The battery pack really isn't that hard to change, just takes a bit of patience & care to remove the front panel & screen & then it's more or less accessible.
The front panel is fixed onto the case by a self adhesive strip that runs around under the panel - plenty of heating with a hairdryer & you can eventually get a plastic blade / edge (plectrums work well) under the side of the panel, work it round & remove the panel. After the panel, the screen comes out & the battery is revealed.
There are plenty of online guides which I would advise looking at as they will show you where any vulnerable cable are around the perimeter of the front panel - make sure it's the correct guide for the model you have though.
If you don't fancy taking it on most independent phone shops (you know the sort) should be able to do the job, or know someone who will (might be worth clearing any personal data, doing a factory restore & removing sim if fitted beforehand though).