How do you change rear end fluid 1971 mustang?

Easy enough, Look on the pumpkin ( big round part in the middle ) there will be 2 plugs, a low one and a high one.

Remove the low one and allow the contents of the diff drain into a pan, while waiting remove the top one.

Once it is all drained, reinstall the lower plug and using a pump start pumping the differential oil into the top hole, when the fluid overflows the top hole you are done, wait for the remaining overflow to drain out the top hole a nd reinstall the plug.