I’ve been trying to switch my 127/150 kh spirit cranks (the ones that come with the KH 24) to 110/137 cranks but the crank bolt seems to tight and doesn’t come out do you have an idea of how to unscrew the bolt?
First off are you turning the bolts in the correct direction? and secondly do you have a long enough wrench?
Usually the removal of the crank bolts is not the main issue. The other issue is that you may require a proper crank puller to get the cranks off the splined axel once the bolts have been removed.
If you do not have the proper tools then you might be further ahead to take the uni to your local bike shop & have them swap out the cranks for you.
If you plan to swich cranks fairly often then you might want to pick up the correct tools for the job.
You may have got some dirt in the threads the last time you installed the bolt. As Harley said you may need to use a longer wrench… once the bolt is out be sure to clean both the bolt and the axle threads with some WD-40 or other solvent before replacing the crank.
What kind of wrench are you using? If you have a socket wrench, find a pipe that fits over the handle of the socket to make it longer…
Use caution when doing this. While it’s very effective, it’s also easy to break bolts when using such an extension. If it’s actually seized, hit it with some penetrating oil and let it sit for a few hours before you break something.
Yup.
Also, if you’re finding the bolt takes lots of effort to turn, screw it out 1/4 turn, then screw it back in 1/4 turn before continuing, hitting it with penetrating oil or WD40 at the same time. This will bust up some of the crud that’s worked its way into the threads.
If you’re not familiar, penetrating oil is sold as “Kroil” or “Liquid Wrench” or “PB Blaster” at the hardware store… but WD40 would work. Heck, even kerosene would do the job in a pinch… just don’t smoke while you’re working! 
WD40, Kroil, and the others are good to penetrate and clean. All of them should be kept away from bearings. Since they are penetrating solvents they can easily get into bearings and ruin them. The solvent action will eventually break down grease enough that it no longer can withstand any high psi loads.