It's a while since I installed on my own PRS505 but I made some notes at the time. This is what I wrote:
Code:
1. Copy directory PRSPInstaller to an empty SD card.
2. Put SD card into PRS505 and switch on.
2a. To take a backup enter "Expert Mode" and choose backup all partitions
Copy files from PRS505 root directory to safe place on the PC.
3. Repeat 2 and follow on-screen instructions.
Edit: Oops! Too slow...