How do I delete files from iCloud?

See this article https://support.apple.com/kb/ph12793?locale=en_US

Delete photos or videos

1. In the Photos app on your iOS device (iOS 8.1 or later), tap Photos at the bottom of the screen, then view your photos and videos by moments.
2. Tap Select, tap one or more photos or videos, then tap .
3. Tap Delete [items].

You can also delete photos and videos using the Photos app on your Mac (OS X 10.10.3 or later) or on iCloud.com. See Use iCloud Photos to store photos in iCloud in the Photos User Guide.

Deleted photos and videos are placed in the Recently Deleted album on your device, where they remain for 30 days. After that, items are permanently removed from all your devices set up for iCloud Photos.

You can remove items from your devices before the 30-day time period is up. On your iOS device, in the Recently Deleted album, tap Select, tap to select one or more items, tap Delete, then tap Delete [items].