How do you remove the front seats from a 1965 Mustang? do you go up from underneath the car?

NO. there will be 4 bolts under each seat, pull the seat all the way forward and remove the rear 2 bolts at each end of the seat track, then push the seat all the way back and remove the 2 front bolts.

My 1965 Mustang, made mid-run in 11965, has four 2" rubber plugs directly under the seat track bolt locations. And under means under the car floor pan. To remove the seat, the plugs must be removed, then a deep socket 1/2" bolt on a ratchet will reach the nut/washer combination and can be removed from the long bolt/stud that is attached to the seat track rails. Jack the car up high and set on jack stands to get enough room to lie underneath the car. Be careful not to drop the nuts between the floor pan and inner pan or you'll have to get a magnet to fish it out. Once the four nut/washers are removed, the whole seat comes up from the floor. To install, set seat track studs back in holes and re-attach bolt/washers from underneath the car.

I have a 1970 mustang and they have to be removed from under the car. Hopefully you dont have anything welded to them like mine did when I bought it.

Yes.