Here, have a guide:

FOR 2.50/2.60:

Get the appropriate ZIP file from the very first post in this thread. password -

I_Use_It_At_My_Own_Risk

(copy and past, password is case sensitive)

There should be an EBOOT.PBP file in the ZIP file.

On your memory stick, go to PSP, then GAME. Make a folder called DOWNDATERTEST.

Put the EBOOT.PBP file in DOWNDATERTEST.

If you have a 1.50 dump, copy the DOWNDATER folder to the TOP LEVEL of your memory stick.

IF YOU DO NOT HAVE A DUMP, DO NOT EVEN ATTEMPT TO RUN THIS PROGRAM. YOU MAY BRICK YOUR PSP.

For example, if your memory stick is O:\, copy it there.

This folder should contain two folders and two files:
DOWNDATER\DUMP\ (containing the 1.50 dump)
DOWNDATER\PRX\ (containing the 1.50 UPDATE .PRX files)
DOWNDATER\inputfl.bin
DOWNDATER\outputfl.bin

IF THIS IS ALL OK, YOU SHOULD BE SET:
* The two files and two folders mentioned above
* PSP\GAME\DOWNDATERTEST\EB OOT.PBP
* eLoader
* GTA
* A PSP THAT DID NOT COME WITH 2.60 PRE-INSTALLED

Now... Go into GTA, go into eLoader, and run "DOWNDATER TEST".

The Memory Stick light will blink for about 2-3 minutes.

Afterwards, your PSP should reboot.

You should get the blue "Reset to defaults" screen. Press O.

The PSP will reboot again. You will have to define basic settings again.

You might notice a "Disc could not be read" error.

If you do, take the battery out, put it back in, and reboot.

Good luck, folks!