Overwriting my local branch with remote branch

first, create a new branch in the current position (in case you need your old 'screwed up' history):

git branch fubar-pin

update your list of remote branches and sync new commits:

git fetch --all

then, reset your branch to the point where origin/branch points to:

git reset --hard origin/branch

be careful, this will remove any changes from your working tree!

What I do when I mess up my local branch is I just rename my broken branch, and check out/branch the upstream branch again:

git branch -m branch branch-old
git fetch remote
git checkout -b branch remote/branch

Then if you're sure you don't want anything from your old branch, remove it:

git branch -D branch-old

But usually I leave the old branch around locally, just in case I had something in there.