Using two flat head screwdrivers, carefully press against the bearing to work it off.

Remove the pinion gear. I put my cutter assembly in a wood vice then carefully used a small pipe wrench to unscrew the gear. It only took a little pressure to get the gear to start unscrewing. If it doesn't come off easily, try WD-40, wait a bit, then try unscrewing again.

Clean the pinion gear and screw it onto the new cutter.

Using heavy gloves, carefully insert the cutter into the housing from the drive belt side. If it doesn't slide easily into the bearing seats, use a soft head mallet to drive it in. Screw the bearing retainer onto the bearing and housing to seat the bearing properly.