It's so simple to do heres a guide:
This will ONLY WORK on a 2.0.
1) You need to unzip the files MPHDowngrader.zip and EBOOT_1.50.zip. Use a program such as
WinRAR, 7Zip, or WinZip (check Google). Once they are unzipped, you're ready to begin.
2) Set up a USB connection with your PSP and your computer.
3) Once the connection is set up, open up My Computer and browse to the removable disk that
is your PSP. You'll need to set a few things up here.
4) When you first double click to open up the PSP on your computer, you'll be in what's
called the root menu. There will be a PSP folder showing if you are in the right place.
5) Copy over to your PSP the following files to THIS location in the root folder...NOT
in the PSP folder or any other folder. Index.dat and h.bin should both be in your root
directory.
6) Once this is done, open up the PSP folder and then open up the Photo folder. Here,
you will copy overflow.tif over, keeping it in the Photo directory and not a sub directory.
7) Now, go back to the PSP directory and go into the Game directory. If there is not
already, create a folder called UPDATE. It MUST be called UPDATE and nothing else.
8) Open up the new UPDATE folder and then copy EBOOT.PBP into this folder from your
computer. Once all of this is done, you're ready to downgrade. So far, so good.
Double check that you followed the above instructions before continuing.
Good.
9) Now, disconnect the USB connection from the computer and plug the AC
adapter into the PSP. As with all upgrades, you must have a certain percentage of battery
life as well as the AC adapter plugged in, or this will not work.
10) Go to the Photo icon on your PSP and scroll through the pictures to get to
overflow.tif. You do NOT have to press anything. It will start automatically.
Two strings of letters/numbers will appear. This is normal and good.
11) Wait AT LEAST 30 SECONDS and then turn off your PSP by holding the power button up
until it fully shuts down. Then, start your PSP again.
12) You will now go over to the Game icon and scroll down to memory stick. Select it
and you should see the 1.5 update.
13) Select it and start it up. Go through it as you've done before with your 2.0 update
and follow the prompts. When the update reaches 99% it will crash and show an error.
THIS IS OKAY. DO NOT PANIC. Once again press the power switch up until it fully turns off.
14) Now, turn the PSP back on. You will get another error. Just press the "O" button
and it will restart after rebuilding the system data. Then just set it up as you did
when you first got your PSP and you have a brand new 1.5 PSP. Be responsible with it.
Go
here to watch a video of it being done.