How to delete iOS 9 install file?
Go to Settings > General > Usage > Manage Storage.

The full file is around a GB but some people have partial files as small  as only 40 MB so You might need to carefully search for your download. 

Tap Delete Update, and tap Delete Update again to confirm.
hope This will effectively remove the downloaded update from your device and prevent you from accidentally installing it. 
if you are going to upgrade in future keep save the shsh blobs (ios 7) so you can downgrade to ios 7 later .