some (dis)assembly required.
I'm going to guess that your MMM is an RC54-21b ? which is probably similar to my RC60-24B.
you have 4 pulleys, center is the drive case and drive pulley for all. just to the rear is the tensioner pulley, left and right are the outer blade drives.
this is easiest to do with deck out from under the tractor, remove the left and right debris shields, blow out all the grass and crap.
standing behind the deck, drive unit in center, you'll need to remove the right side mounting bracket for the drive unit. ( 4 bolts, 2 into case, 2 into deck )
I find it easier to just remove the spring on the tensioner, be careful, it's pretty stout and can bite ya !!
with spring removed, the belt can be removed and new one installed, I'd check the bearings in tensioner pulley, and that the tensioner arm moves freely ( grease the pivot now, so grease gets all the way around the pivot bushing ) check that the clip that suspends the pulley on tensioner arm is in place ( my pulley dropped and was riding on the arm )
install new belt, install spring ( you'll need to be creative here, my way is using a large pry bar to lever the spring into position, YES< I have launched it across the driveway !! )
reinstall gear case bracket, check gear case oil level ( 80w90 ) grease all the blade drive quill bearings grease the drive shaft joints ( just a couple pumps per fitting, too much grease will push the seals out )
mount the MMM on the tractor,
GO mow the yard !
and I grease all the fittings on wheels, casters and blade drives every other mowing, just a squirt or 2 in each fitting
