Be sure that you're downloading over Wi-Fi and not via 3G or 4G, or you may end up using up all your data for the month.
As a general principle this is a sensible idea - you may change your mind and decide you want to downgrade from i OS 11 to i OS 10, and this will be easier if you make sure you've got a copy of the latest version of i OS 10 compatible with your device.
Make sure you plug your device into a power source.
Running out of battery mid-download can foul up the update.
Beware: the backup will include purchased music, TV shows, apps, and books; photos and video in the Camera Roll; and device settings, but it won't include anything you synced from your computer.
To re-sync that stuff you'll need to sync with i Tunes.
While computers such as typical laptops are 'mobile', the operating systems usually used on them are not considered mobile ones, as they were originally designed for desktop computers that historically did not have or need specific mobile features.