How do I delete photos from my iPhone 6?

Were the photos placed onto the phone by syncing from a computer using iTunes? If so, sync again using the directions below from Sync photos from your computer to your iOS device using iTunes - Apple Support :

Delete synced photos

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.

The suggestions from FoxFifth above are very good. I have found out (by trial and error) that if you are using a PC (Windows) it is best to delete the iPod Photo Cache before attempting the above. Go to your Windows File Manager and under Pictures > My Pictures you should see a folder called iPod Photo Cache (iPod is correct, not iPad). Delete it (you will not loose any pictures on your computer). Then go to iTunes and do a sync with no folders selected. You may have to click the box Sync From but do not select any albums below.