Can i install custom firmware again in my psp?

three years back, i went to a psp shop and i asked the salesperson to install some games in it. he installed some games by using m33 and he did not mention about sony update. so the very next day i updated my psp to my shock that none of the games played. now, i hadnt touched my psp for the past three years due to some reasons. now, i want to again install the m33 in my psp to play some games. i searched the internet to find that i need a pandora battery which i cannot afford to buy. then i downloaded chickhen r2 which did not work in my psp either. i have the following doubts; would the salesperson have made my battery a pandora battery before installing? if so, can i install m33 again? can i downgrade my psp from 6.37 to any lower custom firmware version? if i cannot install m33 how to make pandora battery, is it safe to make pandora battery? if so,what are the other safe custom firmware versions which can play iso or cso games withot pandora other than chickhen r2?