How do you change drive belt on mtd riding mower mod 13ax675g062 ser 1d309c30054?

which one? there are two belts.

take off the shift knob and access panel (two screws) remove clip on the varispeed pulley and pull up and remove the top belt which is the rear one and then you can also remove the bottom belt which is the front one, then to remove front belt you will have to first remove deck belt at the engine pulley and then remove the pulley to drop it down so you can remove the front drive belt. the installation is the reverse. the rear belt is kinda harder to install, just get it started on the rear pulley and walk it on, it is sometimes easier to remove the battery and the insert to get at it better. sometimes you will have to do some adjustments but not always. Get the manual somewhere on the web which will tell how to change belt and adjust as necessary.