The 2.71 is downgradable, the 3.03 is not currently.
You first need to check and see if your psp is a hardware model TA-82 or not.
THIS IMAGE shows you how to check.
The best downgrader for a non ta-82 is probably the
DAX 2.71 generic downgrader, but this requires you to do some reading as you will need to install
Hen-D (or C, a previous version) to allow the downgrader to work.
You will need to
download the official 1.50 update (the one you posted in another thread), then extract the eboot.pbp file from that zip file, rename it to UPDATE.PBP and place it in <psp's drive letter>/PSP/GAME/UPDATE as the readme.txt file tells you.
ALWAYS read the readme.txt file and also any info on the downloads page before you use an program.
There is also
THIS 'easy downgrader' which is supposed to automate the process using a different downgrader, I have no knowledge of this one and would strongly recommend spending the small amount of time required to understand how the DAX generic 2.71 downgrader works. The simplest way is not always the best and the things you pick up while reading through readme files or release threads on forums will help you in the future also.
If you have a TA-82 version psp, then try the
Easy TA-82 2.71 downgrader.
Newb here
Try using 
Linear Mode
