How to update sim-locked iphone 4 onto iOS 7?

[iMore Question]

I tried all tutorials and used redsnow/snowbreze but I am unable to update my iPhone 4 to ios 7. the phone is sim-locked (don't use it for call, just for apps and games) and when I start itunes, they recognize it and tell me to enter a proper sim. so I cannot install downloaded ios 7 ipsw.

and if I try to put iphone into itunes mode and then start itunes, they start upgrading but stops each time with error.

I currently have ios 6.1.3 tethered.

 

[iOS Gravity]

Just enter a sim from the carrier its from. It doesn't need to have a plan on it.