The old iOS link is here:
http://www.iclarified.com/750/where-to-download-iphone-firmware-files-from
On a PC you hold Shift and click restore to select the iOS file you downloaded from here.
On a Mac you hold Option.
However, you may not be able to accomplish this. iOS 9 signing servers have been shut down long ago. Unless you saved SHSH blobs using Tiny Umbrella https://tinyumbrella.org/, you may be out of luck.