It's easy to delete the photos and videos that you don't want anymore, just tap the Trash icon. You can also learn how to make space on your device and how to get your photos back if you accidentally delete them.
Make space on your device
Before you delete anything, make sure that you that you still want. When you use iCloud Photo Library and delete a photo or video on one device, it's also deleted on all other devices where you're signed in with the same Apple ID.
To see how much storage you have left on your device, go to Settings > General > [Device] Storage. Learn more about managing your device photo storage.
Keep only what you want
Tap to delete photos and videos from your iPhone, iPad, or iPod touch. When you delete a photo, it goes into your Recently Deleted album. They'll stay in there for 30 days and then they're gone forever.
If you add photos to your iPhone, iPad, or iPod touch from iTunes, you can't delete them directly from your device in the Photos app. Learn how to delete the photos from iTunes.