Hex-edit a signed zip file

I'm having some problems with hex editing. I'm having a signed zip file witch I want to edit without removing the signature.

Actually it doesn't necessarily have to be done by hex editing though..

Well, I want to edit the radio.img in this file (yes, I know it's big) to this file.

As you might understood, it's an update file. An update file to my phone actually.

If anyone could do it for me, it would be appreciated. Or, even better, assist me with it.

Thanks.