How to back up with iCloud

If you want to make an iCloud Backup, go to:

Settings → Apple ID → iCloud → iCloud Backup → enable Back Up This iPhone

To be able to take an iCloud Backup, you must be connected to Wi-Fi. Also make sure you have enough space in iCloud for your backup to be saved.

You can find more useful information here:

How to back up your iPhone or iPad with iCloud - Apple Support