Following the release of iOS 10.3.2, Apple has stopped signing iOS 10.3.1. This means you can’t downgrade to iOS 10.3.1. If you’ve upgraded, you must stay on iOS 10.3.2.
iOS 10.3.2 is a free over-the-air update available to all users with a compatible iPad, iPhone or iPod touch. It can also be downloaded and installed on iOS devices using iTunes on a Mac or PC.