Download it from http://www.psp-hacks.com/forums/viewtopic.php?id=30380
You then start it up, make the correct settings (I probably shouldn't mention them here,) it will then reboot the PSP in 2.5.
In answer to your question, no you don't need a 2.5 UMD in the drive (I use Death Jr.,) but it has to be a game. And you will only need a UMD in the drive if you want to load a game. For the web browser and stuff, you don't need a UMD in the drive.
If you need more help, PM because right now I'm a bit concered the mods won't like this.
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