Replacing the speakers is a pretty straightforward operation. All four speaker openings are for 6½” speakers. Speakers of a different size MIGHT be adaptable. The rear speakers are easily accessed by carefully popping off the grills and removing the three small screws holding each speaker in its frame. In the doors, the speakers are reached by removing the door panels. This is not as hard as it sounds. With a phillips screwdriver remove the screw in the door handle and the screw just below the door lock. The door handle/pocket just lifts right out (it’s a good place to store the seven screws involved in this little operation). Inside the hole left by removing the handle are two more screws - remove these. Carefully pull the door panel away from the bottom edge of the door. If necessary spend the few bucks on one of those little tools for this. the door panel is held on to the bottom edge of the door frame with standard plastic fasteners. Once the door panel’s edge is free, it should just lift right off the upper lip of the door at the window sill. Connected will be a small wiring harness that can easily be unclipped. The speaker is right there and it’s three screws can easily be removed and the speaker replaced. Reassembly of the door panel is a simple reversal of the process. There is also a small plastic triangle at the corner of the window, hiding the screws that attach the side view mirror - this may need to be removed too (or it may just fall off) - it simply slides on and off.
Replacing the existing speaker wire might be in order, depending on your preferences. If your ear is less sophisticated, then the stock wire might do just fine. If, on the other hand, your ear can detect the subtilties of certain hardware upgrades, the replacement of the speaker wires with more substantial stuff might be worth tackling at the same time as the speakers. Can’t hurt, right?