Don't Go Crazy Deleting Files
If you own an 8GB or 16GB device, don't go crazy deleting files.
As a reminder, the iOS 9 update only requires 1.3GB of free space to install on the iPhone 6 Over-the-Air through settings. While it could be different for other iOS devices, we're expecting the differences to be minor.
If you're short on space and you can't even make it to 1.3GB, remember that you can install iOS 9 via iTunes. If you go that route, you won't have to delete anything.