i'v been playing FSX a long time ago, but i don't remember how to do this (missed pics) max heigh: "Overheighted"
This question (or rather, statement) would be more at home on the FSX forums. But im assuming that if you are asking how to use the X-15, download it and install it.
I think you had to go to the map thing or w/e showed your altitude and speed, trajectory and coordinates. And then you just change the altitude value. I might be wrong though, been a long time since i played FSX.