How to delete iOS 9 install file? I have an ipad 2 with iOS 7. I got a notification about iOS 9 ready to install. I don't want to install iOS 9.

The problem is before this I used to have 1GB+ storage space and now I only have 100MB+ storage space left. I'm almost 100% sure the space is taken by iOS 9 install file.

In settings general usage I can see iOS 9 update but the size is only about 40MB. I have deleted it and restarted the ipad but the space recovered is only the 40MB.

Where is the rest of the install file? Can I delete it?
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 .