How to update ios 7 to ios 8

iOS 9.3.5 can be installed on an actual iPhone 4s just fine either through Software Update directly on the iPhone or through iTunes.

If you don't get the option to update to iOS 9.3.5 when going in to Settings->General->Software Update, then you likely have an iPhone 4 (not a 4s) and it cannot be update past iOS 7.1.2.

Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support

As of this date, I don't believe there is any legitimate way to update an iPhone 4s to iOS 8. You may still be able to update to iOS 9.3.5.

you can do it from settings->general->software update

if it says you have the latests then likely you have an iphone4 rather then a iphone4s because iOS 7.1.2 was the last iOS for iphone4