How to replace master branch in Git, entirely, from another branch?

I have two branches in my Git repository:

1. master
2. seotweaks (created originally from master)

I created seotweaks with the intention of quickly merging it back into master. However, that was three months ago and the code in this branch is 13 versions ahead of master.

It has effectively become our working master branch as all the code in master is more or less obsolete now.

Very bad practice I know, lesson learned.

Do you know how I can replace all of the contents of the master branch with those in seotweaks?

I could just delete everything in master and merge, but this does not feel like best practice.

You should be able to use the “ours” merge strategy to overwrite master with seotweaks like this:

git checkout master
git pull
git checkout seotweaks
git merge -s ours master
git checkout master
git merge seotweaks

The first two steps are a useful precaution to ensure your local copy of master is up-to-date. The result should be that your master is now essentially seotweaks.

(-s ours is short for --strategy=ours)

From the docs about the 'ours' strategy:

This resolves any number of heads, but the resulting tree of the merge is always that of the current branch head, effectively ignoring all changes from all other branches. It is meant to be used to supersede old development history of side branches. Note that this is different from the -Xours option to the recursive merge strategy.

Update from comments: If you get fatal: refusing to merge unrelated histories, then change the second line to this: git merge --allow-unrelated-histories -s ours master

What about using git branch -m to rename the master branch to another one, then rename seotweaks branch to master? Something like this:

git branch -m master old-master
git branch -m seotweaks master
git push -f origin master

This might remove commits in origin master, please check your origin master before running git push -f origin master.

You can rename/remove master on remote, but this will be an issue if lots of people have based their work on the remote master branch and have pulled that branch in their local repo.
That might not be the case here since everyone seems to be working on branch 'seotweaks'.

In that case you can:
git remote --show may not work. (Make a git remote show to check how your remote is declared within your local repo. I will assume 'origin')
(Regarding GitHub, house9 comments: "I had to do one additional step, click the 'Admin' button on GitHub and set the 'Default Branch' to something other than 'master', then put it back afterwards")

git branch -m master master-old  # rename master on local
git push origin :master          # delete master on remote
git push origin master-old       # create master-old on remote
git checkout -b master seotweaks # create a new local master on top of seotweaks
git push origin master           # create master on remote

But again:

• if other users try to pull while master is deleted on remote, their pulls will fail ("no such ref on remote")
• when master is recreated on remote, a pull will attempt to merge that new master on their local (now old) master: lots of conflicts. They actually need to reset --hard their local master to the remote/master branch they will fetch, and forget about their current master.

Update/Note 2022:

• master would nowodays be named main
• git checkout is obsolete since Git 2.23 (Q3 2019): use git switch for branch operation.

    git branch -m main main-old  # rename main on local
    git push origin :main          # delete main on remote
    git push origin main-old       # create main-old on remote
    git switch -c main seotweaks # create a new local main on top of seotweaks
    git push origin main           # create main on remote

Since seotweaks was originally created as a branch from master, merging it back in is a good idea. However if you are in a situation where one of your branches is not really a branch from master or your history is so different that you just want to obliterate the master branch in favor of the new branch that you've been doing the work on you can do this:

git push [-f] origin seotweaks:master

This is especially helpful if you are getting this error:

! [remote rejected] master (deletion of the current branch prohibited)

And you are not using GitHub and don't have access to the "Administration" tab to change the default branch for your remote repository. Furthermore, this won't cause down time or race conditions as you may encounter by deleting master:

git push origin :master

I found this to be the best way of doing this (I had an issue with my server not letting me delete).

On the server that hosts the origin repository, type the following from a directory inside the repository:

git config receive.denyDeleteCurrent ignore

On your workstation:

git branch -m master vabandoned                 # Rename master on local
git branch -m newBranch master                  # Locally rename branch newBranch to master
git push origin :master                         # Delete the remote's master
git push origin master:refs/heads/master        # Push the new master to the remote
git push origin abandoned:refs/heads/abandoned  # Push the old master to the remote

Back on the server that hosts the origin repository:

git config receive.denyDeleteCurrent true

Credit to the author of blog post http://www.mslinn.com/blog/?p=772