My stepper getting hotter

Hi guys;
Finally my geckos are running. My stepper almost fly without lose any
step. but I notice that the steppers are getting hot after 45 min and
I am wondering if there is a way to reduce the voltage of my power
supply. It is 65v 4am and I would like to reduce it to 48v. Is there
anything like a voltage reducer or some like that and keep the same
amperes output (4am)?