You should begin by removing the deck from the tractor.
Remove the belts and the mandrel guards from the deck.
Block the blade with a strong piece of wood so it can't turn and remove the nut on the mandrel pulley. This is difficult to do without an impact driver. Don't hold the blade with your hand you might get cut. Use an impact driver to remove this nut and the blade bolt if possible.
Turn the deck over and use the wood to block the blade and remove the bolt holding the blade to the mandrel shaft and remove the blade.
Remove the pulley and the washer from the top side of the mandrel. Use a block of wood and a hammer to drive the mandrel shaft out of the bottom of the housing. The top bearing can be removed by a using a 12 inch broom handle and a hammer. Place the broom handle against the bottom of the bearing and tap it out the top.
Drive the upper bearing into the mandrel housing using a piece of wood on the bearing and tapping the wood with a hammer. The lower bearing is on the jackshaft and will go into the housing when you tighten the nut on the pulley.
Reassemble the blades and mandrel cover and reinstall the deck under the tractor.
The part number for the mandrel is 137646 and can be ordered from the following link.
If you do not feel confident repairing this problem yourself, then you can have it repaired at your home by a Sears technician. Here is a link for the website:
Sears Home Services