Assuming you still have the factory Bose head unit, there's a guide on adding auxiliary input
here.
But the best upgrade would be to replace the head unit entirely with something more modern, as well as all four door speakers since the Bose speakers are proprietary with their own amplifiers; it's a hassle to bypass the amplifier wiring, so you'll want to run new speaker wire to each door.
If you do not want to go through the effort of doing that, it's possible to bypass the wiring as I mentioned above (if you'd like the necessary info I can dig it up), or you can simply use a line output converter between the head unit and the stock wiring to allow use of the head unit without any other modifications.