Today Apple released iOS 9.3.5 with unspecified security patches for iPhone, iPad, and iPod Touch. Since this is likely the last update we will be seeing for iOS 9, I highly recommend that you install it right away. If you’ve been holding off on updating to iOS 9 because you were waiting for it to become more mature, it’s probably not going to get any more mature than this. If you wait until next month your only upgrade option will be iOS 10.
To install the update, go to Settings on your device > General > Software Update.