how do i move my photo library back to iPhoto from Photos?

You can revert to using iPhoto ...

Help here > Revert To Using iPhoto Instead Of Photos

"However, if you rely on an iPhoto feature that is missing from Photos, you can try to revert to iPhoto. If you do revert, bear in mind that you’re only buying yourself a bit of time. iPhoto is no longer being updated, so it’s only a matter of time before it becomes incompatible with some future version of OS X. We don’t know if that’ll be in a few weeks or months or years."

that is not possible

Unless you trashed it you iPhoto library is still there and iPhoto still works (you do need the current version 9.6.1 - if you do not stay up too date see Can't Update iPhoto because it is not available in the App Store )

You will have to export any new photos form Photos and import them into iPhoto

and of course if you want help you can give details here and post a question and get help

LN