Open Settings on your iPhone or iPad
Tap your name at the top
Tap iCloud
Tap Photos
Turn on Sync this iPhone or iCloud Photos
Connect to Wi-Fi
Plug in your device if needed
Wait for photos to upload to iCloud
On Mac, open System Settings
Click your name
Click iCloud
Click Photos
Turn on Sync this Mac
On Windows, install iCloud for Windows
Sign in with your Apple ID
Turn on iCloud Photos
Keep the device connected until syncing finishes