Grab it ASAP before the download servers get jammed up!
To be clear, this is a DIFFERENT update compared to the 9.3 replacement released a few days ago. Everybody should update to this, regardless of their device – even if you got that update a few days ago.
9.3.1 should be available via the Settings app – tapping General > Software Update will trigger the over-the-air update system. If the update doesn’t appear for you over the next few hours, try using iTunes to update: attach your device via USB to your Mac or PC, then sync using iTunes. You should then be prompted to update.