• Make sure your branch is up to date with the master branch.
• Run git rebase -i master.
• You should see a list of commits, each commit starting with the word "pick".
• Make sure the first commit says "pick" and change the rest from "pick" to "squash". -- This will squash each commit into the previous commit, which will continue until every commit is squashed into the first commit.
• Save and close the editor (press Esc key, type :w and hit Enter key) -- save a file and quit vim / Vi by pressing Esc key, type :x and hit Enter key.
• It will give you the opportunity to change the commit message.
• Save and close the editor again.
• Then you have to force push the final, squashed commit: git push --force-with-lease origin.