1996 70 hp johnson outboard ,,need to know why the motor rise up when I put it in reverse and give a little speed to back up, and when I let off the speed the motor goes back down.

In reverse, the propellor is pulling the motor away from the boat, this is the same direction as the motion you get when you tilt the motor up for transport. Check the motor mount, especially the motor tilt mechanism to make sure nothing is excessively worn, bent or loose. Maybe the latch that's supposed to hold the motor in the run position isn't working right - that could be bad news if it lets the motor drop down while the boat is on the trailer.