If you have any files that are not tracked by Git (e.g. uploaded user content), these files will not be affected.
First, run a fetch to update all origin/<branch> refs to latest:
git fetch --all
Backup your current branch:
git checkout -b backup-master
Then, you have two options:
git reset --hard origin/master
OR If you are on some other branch:
git reset --hard origin/<branch_name>