How to squash commits in git after they have been pushed?

Squash commits locally with

git rebase -i origin/master~4 master

and then force push with

git push origin +master

Difference between --force and +

From the documentation of git push:

Note that --force applies to all the refs that are pushed, hence using it with push.default set to matching or with multiple push destinations configured with remote.*.push may overwrite refs other than the current branch (including local refs that are strictly behind their remote counterpart). To force a push to only one branch, use a + in front of the refspec to push (e.g git push origin +master to force a push to the master branch).

On a branch I was able to do it like this (for the last 4 commits)

git checkout my_branchgit reset --soft HEAD~4git commitgit push --force origin my_branch

Minor difference to accepted answer, but I was having a lot of difficulty squashing and finally got it.

$ git rebase -i HEAD~4

• At the interactive screen that opens up, replace pick with squashat the top for all the commits that you want to squash.
• Save and close the editor

Push to the remote using:

$ git push origin branch-name --force