To do this, launch i Tunes, sign in with your Apple ID if prompted and from the Help menu, click Check for Updates.
Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).
Advanced users may wish to update i OS 9 by using an IPSW firmware file.
If you want to force your i Phone or i Pad to check if the update is available, tap Settings If for some reason you don’t want to update over the air, you'll need to connect your device to a computer running i Tunes.
You should update i Tunes to the latest version (currently 12.5.1) before connecting your i Phone, i Pad or i Pod touch.
i OS 10 is available as an upgrade for the following devices: The following devices can't be upgraded to i OS 10: Original i Phone, i Phone 3G, i Phone 3GS, i Phone 4, i Phone 4S, original i Pod touch, i Pod touch 2nd-gen, i Pod touch 3rd-gen, i Pod touch 4th-gen, i Pod touch 5th-gen, original i Pad. Before beginning the upgrade, make sure your i Phone, i Pad or i Pod touch is fully backed up to i Cloud or your computer via i Tunes before performing the update.
It's unlikely anything will go wrong, but if it does, you could lose any data that's changed since your last backup such as new contacts, documents, photos, videos and more.
If you're still on i OS 9 or i OS 8 find out why i OS 10 is worth caring about in our i OS 10 release date and new features article.