How do I update my PSP's 6.38 CFW to 5.50 Prometheus-4?

you cant. if you have a psp 2000 then you can try downgrading to 5.05 and then upgrading to 5.50 prometheus. but if you have a psp 3000, then you are out of luck. you cannot hack it or downgrade it. 6.38 ofw was specially released for covering up these hacks. 6.37 is also the same. you should have consulted some website before you updated your psp