This article covers the process of removing the freewheel on such Yukons.
Use a metal punch and hammer to rotate the lock-ring of the freewheel assembly. Turn clockwise to loosen.

Remove the threaded lockring, pull the cogs, bearings, and spacers off the freewheel body.

Use a bench vice (or channel lock pliers, or vice grips) to hold the freewheel body securely. Turn counter-clockwise to loosen and remove.

Grease the new freewheel threads, and carefully install on the hub motor cover plate. Turn clockwise firmly by hand till tight.
Re-install rear wheel, test ride, and check shifting.