How can i update iphone 4 from 5.1.1 to 6.1.2

I am in 5.1.1. and I am coming across more and more apps or app update versions that only run on iOS 6.x. I downloaded iPhone3,1_6.1.2_10B146_Restore.ipsw and when I try to install it Itunes will say "This device is not elegible for requested build" I don't necessarily want 6.1.3 or higher and much less 7.0.4 I've seen some posts that say to stay in 5.1.1, but why not 6.x?