Q: how do I upgrade my powerbook g4 15-inch?

You can't

A PowerBook with G4 processor can't be upgraded beyond 10.5.8

You cannot install anything later than 10.5.x on a G4 PowerBook.

A PowerBook G4 can't be upgraded past Leopard 10.5.8.

(111014)

Perhaps you could start a new topic in the MacBook ASC area to see

if anyone has ideas about the MacBook's failure issues; could be the

device needs some special attention. Or a diagnostic. An Apple Store

with Genius should be able to test it, usually free; even out-of-warranty.

{Just noticed - all three earlier replies - were within the same minute.}

Good luck & happy computing!

Note, if you choose to upgrade to an Intel Mac be aware of issues with compatible applications and your data: https://discussions.apple.com/docs/DOC-2295 https://discussions.apple.com/docs/DOC-6271