I'm in my local computer's master branch of a cloned master-branch of a repo from a remote server.

I updated a file, and I want to revert back to the original version from the remote master branch.

How can I do this?

Assuming you did not commit the file, or add it to the index, then:

git checkout -- filename

Assuming you added it to the index, but did not commit it, then:

git reset HEAD filename
git checkout -- filename

Assuming you did commit it, then:

git checkout origin/master filename

Assuming you want to blow away all commits from your branch (VERY DESTRUCTIVE):

git reset --hard origin/master