How can I update my iOS

These are the instruuctions:--> Update your iPhone, iPad, or iPod touch but, before proceeding, check if your unidentified iPhone model supports more recent iOS versions, as per this compatibility list:

Here are two different ways to ensure the highest iOS that your device will support is on your device.

Update your iPhone, iPad, or iPod touch wirelessly. Click here --> Update your iPhone or iPad - Apple Support

Here are instructions about how to update your iPhone, iPad, or iPod touch with the Finder on a Mac computer --> Update your iPhone, iPad, or iPod touch with the Finder - Apple Support