Search the forum for a more detailed description (there was really detailed thread about replacing the auxiliary belt, recently)
Basically on the Bravo t-jet (will be 99% same on yours) you need to do it from underneath in order to get enough movement,, remove the under-body dust /skid pans, you need a long 13mm box end wrench, you stick it on the tensioner bolt, and pull down, keep it tensioned with one hand and remove the belt with the other. Release the tension(but you might want to keep the wrench on the bolt, find some way to prop it in place), finish removing the belt, clean the roller, check for play/noise (replace if necessary), put on the new belt on most pulleys, pull on the wrench again, and finish installing the belt.