how to turn on back the icloud backup

Greetings!

1. Connect your device to a Wi-Fi network.
2. Go to Settings > [your name], and tap iCloud.
3. Tap iCloud Backup.
4. Tap Back Up Now. Stay connected to your Wi-Fi network until the process ends. Under Back Up Now, you'll see the date and time of your last backup. If you get an alert that you don't have enough iCloud storage space to complete the backup, follow the onscreen steps to upgrade your iCloud storage.

You can find more information on this page: How to back up your iPhone or iPad with iCloud - Apple Support