How to move a branch backwards in git?

If there are no commits on branch A, then the git reset --hard B solution given by Bram Schoenmakers will work.

However if there are commits are branch A which must be preserved, then the following should do the trick:

1. Make a backup copy of your repo (just in case)
2. git checkout A
3. git rebase -i --onto B SHA1-A^

...where SHA1-A^ is the commit id of the parent of your branch A

See the git rebase man page for details.

NOTE: This will rewrite history (as rebase always does). Special consideration should be made if your A branch was ever pushed to a public repo.

Use the reset subcommand:

git checkout A
git reset --hard B
git push --force github

As a sidenote, you should be careful when using git reset while a branch has been pushed elsewhere already. This may cause trouble to those who have already checked out your changes.

I usually use this sequence and find it the simplest way:

git checkout B
git branch -f A B

Delete the branch both locally and remotely, recreate the branch, push the branch back up to the server.

git branch -d A
git push origin :heads/A
git branch B A
git push origin A:A

Alternately you can use the following command to undo that last commit.

git revert c4

Which will make your timeline look like:

c1--c2--c3--(B)
             \
              c4--(C)
               \
                (^c4)--(A)

where (^c4) is a commit that undoes c4

I don't recommend using rebase or revert on a branch that has been pushed to a remote repo, they can cause tons of trouble for you or anyone else using that repo.