unless its still under warantee then just take it to your local watch shop and let them change the battery. battery watches don't really need to be serviced unless there is something wrong with it, or you really have money to burn by letting them clean it (but they do a very good job of cleaning it)
the only difference between your local shoe shop repair type place and breitling is breitling will guarantee its waterproofness afterwards and may give you an extra years warantee, which is a pretty expensive way to get one
Think I paid 25chf for a battery change for my Tag, instead of the 350chf tag wanted.
eta - my tag was still waterproof afterwards