Iphone 4s upgrade to ios 9 help

[thebulls]

Hi guys

i have an Iphone 4s which on ios 7.1.3 but i need to get the latest firmware but it wont let me update i have already done a factory reset and it still says software up to date i know this version does go on a iphone 4s as its on my husbands any help would be gratefully received

 

[DOA]

Basically an iphone 4s only goes upto 7.1.2-7.1.3 it is upto date