While it is possible to cut through the materials of a mattress, we don’t advise cutting your mattress. Inside there are several layers of foam, latex, and coils. And it's nearly impossible to cut evenly through them all. Also, sealing the mattress back up in a way that keeps it from sagging on the cut side is very challenging.
Lastly, cutting up your mattress would void its warranty. This lack of protection will leave you with no defense to easily replace your mattress. It’s best to simply purchase a new mattress that is the size you need.