How to replace cartridge for this tub valve?
The valve cartridge is threaded into the valve body. The valve body stops at the top of those visible threads in your photo. That is also where the valve cartridge begins. 

Get a wrench on the flats of the valve cartridge and turn CCW. Don't use an adjustable wrench if you can help it. Use a fixed box wrench if you have the right size. Brass is soft!

I would really try and remove the valve body from the jacuzzi first to eliminate any risk of damaging the jacuzzi. By leaving it in place and turning CCW on the valve cartridge you'll be tightening the valve body against the tub and things could break... There has to be a way to get it out - how was it installed? If you can get it out hold the valve body with a pipe wrench and unthread the valve cartridge from it as described.