I would start with the front speakers. The rear speakers are less important, but given the age there might not be much left of the paper cones. You may be able to inspect the speakers with a flashlight so you don't have to take the door panel off.
This video shows how to remove the door panel:
https://www.youtube.com/watch?v=3hCRH0giQoU - remove the screws as shown, then start pulling on the panel from the bottom.
The speakers are riveted to the door, so you will need to drill them out and replace with bolts through new speaker mounts which will accept screws for your new speakers.