Futon topper is a type of sofa that extends into a bed and then back to a sofa again at will. Usually the futon is a little more than a metal frame and a mattress. The mattress can be covered in sheets at your own risk.
Remove all sheets that cover mattress on your futon and wipe them in a regular dryer. If you do not have a hair dryer in your home or apartment, find a laundry room in your area. Wear your futon outside on a sunny day. If the mattress on your futon is detachable, separate the base of the futon from the mattress and just carry the mattress outside. This benefits the mattress in two ways: the first is that direct sunlight will help dry out the mattress. The second is that the air circulating outside will also blow against the mattress and help dry out. Leave the mattress outside for a few hours.
Use a device that circulates the air, such as a hair dryer or a fan, to blow air directly onto the wet parts of the mattress. If you use a fan, angle the fan down toward the futon and allow the fan to blow until the unit is dried. If you use a hair dryer, you may have to sit and hold the dryer for a while before the futon gets dry. Both of these will effectively dry out the foam on the inside of a futon mattress.