Things You'll Need
Instructions
Download the firmware from http://dl.qj.net/Custom-Firmware-5.50GEN-D-PSP/pg/12/fid/27829/catid/163 (see Resource section below). It is assumed that your PSP is downgraded or has a running homebrew on the system.
Turn on your PSP handheld system and connect it to your computer with the USB cable through an open USB port.
Unzip the firmware file to the root area of your PSP memory stick. The memory stick should have 550.PBP in its root.
Remove the PSP system from your computer, unplugging the USB cable in the process.
Go to the "Game" folder tab in the PSP and run "PSP Update ver GEN. click the "X" button to do this.
Choose the "Flash Install CFW 5.50 GEN-D" option.
Follow the instruction wizard when the update begins. When it completes, your PSP will be running 5.50 GEN-D.