Undo a Git merge that hasn't been pushed yet
With git reflog check which commit is one prior the merge (git reflog will be a better option than git log). Then you can reset it using:
git reset --hard commit_sha
There's also another way:
git reset --hard HEAD~1
It will get you back 1 commit.
Be aware that any modified and uncommitted/unstashed files will be reset to their unmodified state. To keep them either stash changes away or see --merge option below.
As @Velmont suggested below in his answer, in this direct case using:
git reset --hard ORIG_HEAD
might yield better results, as it should preserve your changes. ORIG_HEAD will point to a commit directly before merge has occurred, so you don't have to hunt for it yourself.
A further tip is to use the --merge switch instead of --hard since it doesn't reset files unnecessarily:
git reset --merge ORIG_HEAD
--merge
Resets the index and updates the files in the working tree that are different between <commit> and HEAD, but keeps those which are different between the index and working tree (i.e. which have changes which have not been added).
Assuming your local master was not ahead of origin/master, you should be able to do
git reset --hard origin/master
Then your local master branch should look identical to origin/master.
See chapter 4 in the Git book and the original post by Linus Torvalds.
To undo a merge that was already pushed:
git revert -m 1 commit_hash
Be sure to revert the revert if you're committing the branch again, like Linus said.