Save a full iCloud backup of your iPhone so your photos, messages, and apps are safe.
iCloud backups only run over Wi-Fi, and a charged or charging phone won't time out mid-backup. The first backup can take a while, so plugging in is worth it.
This is your Apple Account banner. From there tap "iCloud" to see everything iCloud is storing for you.
On newer iOS it's under iCloud > "iCloud Backup." Toggle "Back Up This iPhone" on. While you're here, you can also turn on "Back Up Over Cellular" if you want — most people leave it off.
Keep the screen on this page until it shows a completed time and date under "Back Up Now." Free iCloud accounts include 5 GB — if you get an "out of storage" message, you'll need to upgrade iCloud+ or back up to a computer instead.