Depends on the seal, as to how you go about it. If the donk is on the stand, loosen all the main caps.
There are 3 types of seal. The factory 2 piece 'rope' seal, this will need a pin in the main cap to stop it spinning. The aftermarket 2 piece rubber jobby, Install with 10mm sticking out on one side, opposite on the cap, so the joins don't line up. The one piece oil seal (like a timing cover seal) but requires block machining.
Rope seal must be squeezed in to the block groove, usually I use a big ass socket like a rolling pin. Can't see you doing that without removing the crank.
"May" be able to slide the oil seal in.