Downgrade package from backports back to stable

The easier way is using the release option on apt, example:

sudo apt-get install tmux/stable

or, in case you are using the name of the release instead of the tier (ie. squeeze, jeesie, sid instead of stable, testing, unstable) you should use that name instead:

sudo apt-get install tmux/squeeze

This will install the latest version available in the specified suite (stable, testing, unstable, stable-backports, sid, etc.)