Grab the ipa file (via iTunes), transfer to iDevice, and use IPA Installer's "Force Install" option
But now that I check, it has never been updated for iOS 9, so it may or may not work
Not sure if Filza's IPA installer has the option to override any firmware version checks
I had to reinstall an older version of an app once. This was back on the iOS 6 jailbreak.
iOS 7 released and the dev of this app updated but didn't bother mentioning in his description that he updated the app to iOS 7. At the time my iPhone 5 was on iOS 6 (jailbroken) so this broke the app.
After a few support go arounds where the dev ultimately stopped helping once he found out I was jailbroken I used iFile to zip the older version of the app on my 3GS, transferred it via Dropbox to my iPhone 5 and unizipped it inside the applications folder. Doing it that way kept the file permissions intact.
Restarting my phone made the app appear on the homescreen.
I emailed the dev after that to tell him he'd never get another dime from me (this app was an app I bought).