This is a video on how to fix the updating problem on the PSP Custom Firmware (CFW) 5.50 GEN or Prometheus 4. If you ever get the error that says your running on 9.90 and there is no need to update, well heres how to fix that problem.
Step 1: Take your PSP and hit the SELECT button on the XMB and your VSH menu will appear. Change your USB device to "flash 0" and then hit SELECT again to leave the VSH menu.
Step 2: Plug in a USB cable and activate USB mode on the PSP and go to your My Computer (windows XP) or Computer (Windows Vista/7) and locate the removable drive for your PSP.
Step 3: Locate the vsh folder and open it and then inside that folder locate the etc folder and open that and you should see a version.txt
Step 4: open the version.txt document.
Step 5: the first set of number should say "9.90" if you are having this problem. Change those numbers to "5.50" and then save and exit the version.txt document.
Step 6: exit USB mode and go back to the XMB and hit SELECT and change your USB device to Memory Stick (MS) and SELECT to exit and save and your done.
Now you can Update no problem :p