problem updating to ios 9 but stuck on ios 7.1.2 on iphone 4S

I bought my iPhone 4S over eBay and is stuck on ios 7.1.2 and I am trying to update to ios 9 or any recent update. I've downloaded the IPSW file for ios 9 for the 4S on this site. When I press 'shift' and 'check for update' on iTunes the file comes up, however when I click on it, it comes up with 'the firmware is not compatible'. I've tried all the tips including updating itunes, updated computer updates, reinstalled itunes, disabled antivirus software, list goes on but still getting the same message and not able to update. Any help will be appreciated! Cheers

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