See if you can get a parts breakdown on MTD Products Landing Page
. The serial number is required to download a manual, so I can't look. I'll look if you find the diagram and post the s/n, or send it to me via pm.
In general, with any belt replacement, you have to expose the pullleys and you may have to remove the blades.
Blade removal is usually done by wedgeing the blade with a block of wood. Anti-Seize compound when re-assembling will make the job MUCH easier next time.
Either the belt will have a tensioner or there is a non-stationary part which is mounted on a slotted base.
Typical of these is engine to pulley systems. The engine is mounded on a slotted base.