I want to back up my iPhone 7

Your iPhone 7 can be updated to iOS 15.6.

Go to Settings > General > Software Update.

Update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone, iPad, or iPod touch - Apple Support

Update using your computer -

Update iOS on iPhone - Apple Support

To back up your iPhone, open Settings and search for Backup. It will take you to a page where you can select Apple ID > iCloud Backup. This should be on for automatic backups but you can also tap the option to Backup Now for an immediate backup.

As for upgrading the iOS, the highest your iPhone 7 will go is iOS 14. To update, open Settings > General > Software Update. That should allow you to update to the highest version available for your device.