You will need to download the
DAX 2.71 generic downgrader to get your psp back to 1.5.
Never run the updates on your psp, they do not need to be installed to be used in devhook.
The easiest way to get devhook set up would be by using
an automatic installer like this one.
As it says on the download's website:
"Even though it's an easy installer, you'll want to make sure you know how to use these things or you could end up messing your settings. This is especially noteworthy as the installers don't actually come with a readme, so check their site or our forums for additional info on manual installation or on using this easy installer."
ALL downgraders and even some versions of devhook write to the psp's internal flash memory. It is unlikely if you know what you are doing, but still possible you may brick your psp. Neither I, the author of the downgrader, the author of the auto installer, qj.net nor psphome take any responsibility if you damage your psp in the process of downgrading.
Having said that thousands have used these apps with very few issues. Always read up on programs (usually through their release forums on certain sites) well prior to using them.