There is only one belt on your car it drives the water pump and alternator, it should be changed at the inspection 2 service.
The belt will stop squealing when it hot as rubber contracts as it warms up and will also become more playable.
If it's starting to squeal it needs to be changed really, it's not going to get any better. It could also eventually cause damage to your car.
While the belt is off get the belt tensioner checked, to make shore it's working correctly, a bad tensioner would cause the belt to squeal as well. But the belt is the cheapest option and normally the problem.
If you buy one from bmw there Ł20 ish or go to a motor factors there about Ł10 ish
It takes about 15-20 min to change the belt, but if you have small hands it takes 5 min.
Hope this helps