Can anyone tell me the best way to clean my backdrop?

Honestly the best and easiest way to do it is get a light cleaner(even something like windex would do or regular soap and water) and just lightly damp a cloth or mop and clean it off. I wouldn't press to hard as you might have a chance to rip it no matter how heavy duty but just a few swipes of the cloth and soap/water and it should be nice and clean. Just don't use too much water so it can dry and evaporate quickly avoiding any damage to the backdrop.