Never had to do it but as I understand it this is what you do, disconnect heater core hoses, pull glovebox, remove bolts from around the cover plate, remove cover plate, remove heater core.
No need to remove dash or anything else.
What lazy k said. I add make sure the seal is good for the box. I replaced mine last summer and come winter I had no heat. It was blowing out of the box onto the floor. My new heater core did not come with the foam sticky strips, but my friend's new heater core did and he gave them to me since his seal was still good. You can get the sticky foam from the hardware store.