Welcome to MR, sorry to hear from your problem.
As long as it comes up with the "Firmware update did not complete successfully Please try again"-Message, all you have to do is
A) get a Windows PC
B) download http://prs-plus.googlecode.com/files...taller_505.exe
C) connect your 505 to the PC
D) run Uninstaller_505.exe follow instructions on screen.