How do I delete all the photos off my iphone 5c at once?

Start whatever camera application you have on the computer. On a Mac you can use Image Capture. On a PC use whatever application you have to download photos from a camera. Start the app and connect your iPhone. Select your iPhone in the camera application. Select all photos and then click on delete.

The fastest way to delete them, open the photos app, enter the Moments view, tap Select, tap Select to the right of each of the moments (which will select every photo in the moment), then tap the trash icon.

I had to do them on e at a time but that's okay. at least I got them off finally.