How do you get the head off the troy bilt string cutter? I'm try

If by "tearing it up" you mean that little retaining ring inside the main body of the head, that's correct. It must be destroyed to remove the main body.

Very very few of these trimmers can use a different mowing head without suffering damage to the gearbox, driveshaft or centrifugal clutch. (These are VERY LIGHT DUTY trimmers.)

As for the advice you seek:
"Don't run any other head on this unit with expectations of the trimmer holding up."

Troy-Bilt offers other attachments for this engine to use in performing other types of jobs.

At my shop we see a huge percentage of these units with failed drive shafts and gearboxes.

Probably not what you wanted to hear, but it's better than telling you the wrong thing and then you end up being unsatisfied with the solution I offered. I truly wish you good luck.

Hope this helps,
Dolf-

I do not know the exact model but I do know that most of them have a left hand thread and that really stuffs most people up, (me included some moons ago :) )
Usually there is a hole at the top of the string head/ bottom of the shaft, where you can insert a metal pin, turn the head while you push it in until it goes into the head and locks it from turning. Then try to undo it the way you would normally tighten something.

Don't know if this will help but I hope so.
Left hand threads still catch me out now & again, even after decades of machinery repairs :)