Two questions:
1) Is significant surgery required to upgrade an implanted commlink?
2) What book & page(s) have the rules for upgrading implant commlinks?

I was just looking at that myself (My comlink needs an simsense accelerator and a better response)

Well, i would count it as an implant repair surgery. Also a Comlink (internal) needs essence 0.2. That puts it at Cybertechnology+Logic(variable, 1 hour) Surgery Damage: 3S. (Augmentation, Page 176)

I assume that your comlink replaced part of your skull for most components. (The compononents you need to replace/fix) So just cut up the skin, work on the board, flap back the skin, apply skin-glue ™ and be done)

Costs would be for a normal comlink, but i think you need specially designed parts, because of the unusual shape. (Not really unusual since all kinds of people have internal comlinks).

For a standard person with an implant I agree with the above, however I have to point out that a some people will have a full cyberskull to mesh with the commlink. With a cyberskull it would probably be a standard repair test or, if you have it moduled out, a plug and play operation (a la Data's side hatch). Heck a hacker/rigger with a medical drone could probably jump in the drone and do surgery on his own head, which now that I think about sounds kinda cool.

Or the hacker could have the 'Link in his cyberhand/foot