If you are running official firmware, you can't directly upgrade to 5.50GEN-D3. This guide is only for 1000 and 2000 units with 3.71m33 or higher and ta-088v2 motherboards or lower.
1. Connect your PSP to your computer.
2. Download 5.50GEN-D3 here.
3. Unrar 550gend3.rar
4. Copy "UPDATE" folder to ms0:\PSP\GAME folder.
5. Copy "550.pbp" to the root directory of your PSP.
6. Disconnect your PSP from your computer.
7. Plug in your charger.
8. Go to GAME section on your PSP and run "5.50 GEN-D3"
9.Select "Enter", then select "Flash Install CFW 5.50 GEN-D3".
10. Wait for the flashing to finish.
11. Restart your PSP.
12. Check system information if you have 5.50GEN-D3.
13. Enjoy!