Synthetic hair is washed quite differently to human hair, and the good news is that it is easier — the fibre holds its own style, so it comes back to shape on its own.
Use cool water, never hot. Add a small amount of gentle shampoo made for synthetic hair, and let the piece soak rather than rubbing or scrubbing it. Rinse gently, in the same direction the hair falls. Squeeze the water out rather than wringing or twisting, then lay the topper on a towel to take out the excess.
Air dry on a stand, away from direct heat and sunlight. Do not brush it while it is wet. Once it is dry, the style will have returned by itself.
How often depends on how much you wear it — every fifteen to twenty wears is a reasonable guide, or sooner if you have been using product.
EDIT ME: name the shampoo and conditioner you recommend and link to them. You stock hair care already, and this is the right place to mention it.