Assuming you have experience with cambelts and DOHC engines, it's certainly possible to change the 16V TS cambelt in your own garage. However, it's quite a big job with all the different pulleys, locking the cams, job setup, disassembly and assembly. If the 156 has A/C, the cambelt is buried between the splash guard, the engine block and various belts, covers and pulleys (alternator, water pump, cams, A/C pump etc.).
I'd have it done at an authorised garage. They'll give him a warranty in case the head crashes because of the cambelt job.
So, it's a big job if you want to do it right - i.e. change everything instead of just the belt and the involved pulleys.
Have you seen
Jaap Bouma's cambelt guide?