Setting your branch to exactly match the remote branch can be done in two steps:
git fetch origin
git reset --hard origin/master
If you want to save your current branch's state before doing this (just in case), you can do:
git commit -a -m "Saving my work, just in case"
git branch my-saved-work
Now your work is saved on the branch "my-saved-work" in case you decide you want it back (or want to look at it later or diff it against your updated branch).