hi
welcome to the forums
is it just worn? does it screach when starting up cold?, when cold/off how much free play is there in the longest stretch of belt, if its more than about +/-10mm then it needs tightening up a little.
have you any idea how many miles since the timing and aux belts have been last replaced??? it might be worth getting both replaced(this is more a garage job to replace the main timing belt.)
(5years/75k is service life of the belts)
a new auxillary belt is about £15 to buy. main timing belt is not much more.
do you have a 2.0L 8v F3R engine, if so its only a allen key and 21/22mm spanner (offset ring spanner is best) needed to do, look for the tensioner pulley just below the alternator/pas pump.
the allen key tightens the pully(locks it in place), socket adjusts pulley tension on the belt. the allen key is reverse threaded.
do make sure its fully tightened and you have removed the spanner/allen key before you go startting the engine, it can make rather a mess of things if you forget