What's the easiest way to fix these deck joists?
I've replace a few on decks and never had to remove all the planks because there was enough room underneath to get to the joist hangers.
You should remove the screws from the planks going into a joist you want to replace. Then support the joist and remove the hanger from one end and the screws from the other hanger into the joist. Then lower and remove the joist. Take the new joist, work it into the one hanger and the install a new hanger at the other end. You might have to use new hangers at each end if the new joist in a slightly different size than the old one. Then re screw the planks into the new joist.
If you don't have the clearance under the deck to do the above, then you'll have to remove the planks to get to the joists.