Instructions are on page 24 of the owners manual.
You have to remove the deck first.

There's a good chance it's the transaxle.
Sometimes an oil change can help-
Click the link for the PDF file about this-

Here is a link that might be useful: Oil change

This post was edited by bill_kapaun on Sun, May 25, 14 at 15:45

One thing to also check on an AYP rider is the presents of lots of grass on the frame area around where the clutch pedal pivots. Kinda hard to explain, but look where it travels through the frame and at times you will find so much grass built up, it keeps the pedal from fully retracting so full tension is not on the belt. Also on yours, set the park brake an spin the front V idler /listen or feel for roughness in the bearing as these fail often enough which leads to belt glazing. Belt install is easy if you just work through it. Off the front pulley and let it rest on the stack, out of the idlers and any holders then over the fan and out it comes. Reverse to install.

Thanks for the good responses. Research shows that the transaxle is Tuff Torq, perhaps K46BN. Craftsman number 404047. This is not a low cost unit!
Any idea of the typical operating life of these units? Mine has less than 300 hours and the rest of the mower is in excellent condition.
The mower has never been used for towing anything and no operator weighs more than 200 pounds.

OP - if you have hills/inclines and they are =>10 or 15 degrees. the tranny's oil has prolly been overheated and needs a changeout. See manual for servicing instructions.
If your tranny is of the "sealed for life at mfgr" type, then the tranny unit will need to be removed from from the tractor, turned upside down to drain old oil thru top vent (a slow process), refilled with fresh (again, slow), tranny remounted and cycled to remove entrapped air.
Other things to look at are the ground motion drive belt and related pulleys for wear/tension, etc., tranny fan blades not broken off in addition to the good tips that Tom already supplied you.

About this hydrostatic unit, I found an old thread dating 2007 on another forum about the Tuff Torq K46 unit. So this unit is not a new design. Lots of posts about problems. Some even claim the mower makers who use this unit have a design life of 250 hours for their mowers-- I do hope that they are wrong about that.
The K46 is used on huge numbers of mowers. Stopped to see my neighbor who has always used Simplicity mowers. His Regent 20 hp has almost 500 hours and much to my surprise, it has the K46 transaxle. Some John Deere models use it too.
Some posts suggest that certain internal parts can be changed to convert it the K57 model.
Does anyone know what is the heaviest-duty Tuff Torq unit that is a direct interchange, that will bolt right into my mower?

If its not the belt,i would inject a heavy wt.stablizer or motor oil 60wt.,into the breather tube 1\4 qt. at a time.Run it with a container attached to the tube to catch possible over flow.The higher viscosity should make a worn pump push oil better.

New belt now installed on the Craftsman -- of Chinese origin. An unwelcome surprise from my internet purchase. When I cranked the mower this morning, it seemed to operate perfectly normally. That was an encouraging sign.
So it was moved to the work area, deck removed and the belt changed.
My first ever deck removal or belt change on an AYP unit. MUCH easier than on the old Murray I have. I did earlier replace one spindle on the Craftsman without removing the deck.
The Murray required removing the steering gear and shaft to remove and replace the belt-- what a pain!
The old belt was just slick but otherwise in good shape. I drove the mower up a steep hill at low throttle with the new belt and it pulled just fine.
The transaxle unit fan was in good shape and very little trash was accumulated there. The deck and most of the rest under the mower was piled with grass clippings. Got a face full before using the air hose.
Perhaps the transaxle will get an oil change at the end of this mowing season.