Why won't my iPhone 4s update to iOS 8?

[iMore Question]

Why won't my iPhone 4s update to iOS 8

My sister's iPhone has ios 7.1.2 and it wont update to iOS 8 please help

 

[Fit24]

Re: Why won't my iPhone 4s update to iOS 8

Are you receiving a message when you try? If so, what does it say?

 

[Rob Phillips]

Re: Why won't my iPhone 4s update to iOS 8

Welcome to iMore! No version of iOS 8 has been signed by Apple in almost a year. You'll have to update to iOS 9.3.1.