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If you find a download link to beta 1, you probably can if you remove all iTunes 10.5 beta 2 references. But, since the library was updated in beta 2, you won't be able to open iTunes if you downgrade.
 
Are you using osx? If so its pretty easy actually.
First you do need the beta1 file. Then just delete the iTunes.app file in applications folder(if you don't you get an error) install the beta1 and restart for good measure. Then it should work. I had to do it to sync my beta1 iPhone before upgrading to beta2. My library and everything was fine.
 
Are you using osx? If so its pretty easy actually.
First you do need the beta1 file. Then just delete the iTunes.app file in applications folder(if you don't you get an error) install the beta1 and restart for good measure. Then it should work. I had to do it to sync my beta1 iPhone before upgrading to beta2. My library and everything was fine.

It's going to give you an error saying that the library was created in an earlier library. I know, because I've actually tried it.
 
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