Open Settings on your iPhone or iPad
Tap your name at the top
Tap iCloud
Tap Photos
Turn off Sync this iPhone or iCloud Photos
Choose Download Photos & Videos if you want to keep a copy on your device
On a Mac, open System Settings
Click your Apple ID
Click iCloud
Click Photos
Turn off Sync this Mac or iCloud Photos
On Windows, open iCloud for Windows
Sign in with your Apple ID
Uncheck Photos
Click Apply
If prompted, choose whether to keep or delete photos from the device
If you want to stop using iCloud Photos on all devices, repeat the steps on each device