Apple iOS 10 is designed for the iPhone 5 or later, iPad 4th gen or later, iPad mini 2 or later, iPad Pro range and the 6th generation iPod touch or later. Users are automatically prompted to upgrade, but if you don’t yet have a notification you can manually trigger it by going to Settings > General > Software Update
As always, the size of iOS updates differs significantly depending on your device. In the case of iOS 10 it is a major release and typically weighs in between 600MB and 1.2GB.