I go with Mark Longair's solution and comments and recommend anyone reads those before acting, but I'd suggest the emphasis should be on
git branch -f <branchname> <commit>
Here is a scenario where I have needed to do this.
Develop on the wrong branch and hence need to reset it.
Cleanly develop and release some software.
Mistake: Accidentally stay on the release branch while developing further.
"OH NO! I accidentally developed on the release branch." The workspace is maybe cluttered with half changed files that represent work-in-progress and we really don't want to touch and mess with. We'd just like git to flip a few pointers to keep track of the current state and put that release branch back how it should be.
Create a branch for the development that is up to date holding the work committed so far and switch to it.
git branch development
git checkout development
Now we are in the problem situation and need its solution! Rectify the mistake (of taking the release branch forward with the development) and put the release branch back how it should be.
Correct the release branch to point back to the last real release.
git branch -f release release2
The release branch is now correct again, like this ...
git push -f <remote> <branch>
is well described in another thread, though the word "overwrite" in the title is misleading.
Force "git push" to overwrite remote files