Depending on the model and year of the dough sheeter, the detailed instructions may vary.

Watch the general walkthrough here: https://youtu.be/NVbI7J9cSro

Begin by removing the back cover on the dough sheeter by removing the screws holding it.

Once the back cover is removed, you will need to remove the large black nut on the lower right side or lower left side depending on which roller needs to be removed or which conveyor you need to replace. On some machines a large socket is required to do this properly, on other machines the large nut may be slotted, allowing the use of a large flat head screwdriver and hammer to gently tap the nut counter-clockwise to slowly unscrew/loosen the nut and spring washer.

Once the nut and spring washer is removed, you can loosen the roller holder that is connected to the conveyor roller by rotating the roller holder counter-clockwise. On some machines, this roller holder may be slotted, allowing the use of a large flat head screwdriver to loosen the roller holder.

Once roller holder is loosed enough, that will allow enough clearance to pull the conveyor roller out of the machine.

Once the conveyor roller comes out, you can then remove the four screws that connect the table to the housing and you can remove the belt. 

Once you replace the belt, you can simply just do the steps in reverse: re-attach the table back onto the base, put the conveyor roller on and ensure that the driving pins are aligned correctly, rotate the roller holder clockwise to secure the roller onto the machine, put the large black nut and spring washer back on and tighten, and then lastly securing the back cover back onto the dough sheeter.