Updating iphone 5 with ipsw file I have an iPhone 5 with iOS 6.0.1. 
Can I update with an ipsw file to iOS 6.1.2?
I don’t want to have iOS 7. 

Or can I only update automatically to iOS 7?
As you can see here that version of iOS ins't signed anymore by Apple. It means that you can't install that version unless you have saved the SHSH blob with softwares like TinyUmbrella. You can save SHSH blobs for certain firmware until it is signed.