can I downgrade my iPhone 4s iOS 9.2.1 to 9.0.2? I try using ipsw but it isn't signed at this moment. Can I create custom ipsw 9.0.2?  Its possible to downgrade my iPhone?  Thanks.
I'm trying to downgrade my iPhone from the current iOS to an older one, as per .  But every time I try, I always receive this error:     The iPhone could not be restored. This device isn't eligible for the requested build.     I have tried many times with and without DFU mode. Any ideas regarding this?
I am studying General Relativity and some basic QFTs. It bothers me a lot that different books use different metric signatures, i.e. $(-+++)$ and $(+---).$ Can anyone tell me the advantages of using both of them, or why some people prefer the former while some prefer the latter? I would like to know which one should I while tackling different problems.