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.
Automatically back up with iCloud Backup
Make sure that iCloud Backup is turned on in Settings > [your name] > iCloud > iCloud Backup.