How to change the remote a branch is tracking?

Using git v1.8.0 or later:

git branch branch_name --set-upstream-to your_new_remote/branch_name

Or you can use the -u switch

git branch branch_name -u your_new_remote/branch_name

Using git v1.7.12 or earlier

git branch --set-upstream branch_name your_new_remote/branch_name