How do I get my 6 yr old dog to stop pissing in cage

HI, oxman,
Give it the opportunity to go outside at frequent intervals whenever you think it might be time, and reward it with attention or treats whenever ot goes outside.
Call the Vet for an appointment, see if it's a medical issue; possibly a urinary tract infection.
Is it drinking more water than usual? Could be a sign of infection.
Persistence is the key word here. Take it outside frequently, and eventually it will get the idea.
I do wish you the best, and good luck.