$ git checkout filename.txt
blows away whatever changes you've made to a tracked file and replaces the file with what was present in the most recent commit
$ git reset --hard HEAD
forcefully rewinds you to the most recent commit, blowing away all uncommitted changes you've made locally
$ git reset --hard f772f4 (sha)
same as above, but allows you to rewind to a specific named commit (via its SHA)
$ git push origin branch --force
allows you to rewrite history on a remote. you should never execute this command on a branch that anyone other than yourself is tracking.
$ git push upstream master
you have permission to do this, but we are trusting you to do your work in your own branch and submit Pull Requests instead!