Q: how to delete photo library?

You may want to reply with more details of what it is that you are trying to do as it is not clear.

One possibility: if you used iTunes to sync photos to the phone and now want to delete them from the phone, see the instructions below from Sync photos from your computer to your iOS device using iTunes - Apple Support

1. In iTunes, locate and click on your device.
2. Click Photos.
3. Choose "Selected albums" and deselect the albums or collections you want to delete.
   To delete all synced photos, deselect "Sync Photos from" and, when asked, click "Remove photos."
4. Click Apply.

Learn how to delete photos and video synced from iTunes when you can't access the originals.