the easiest way is with umbrella.  the hardest part is to put your phone in restore mode and plug it into a computer to get it’s unique id.  after that, you reboot your phone normally and put it away. it is never modified.  Umbrella is run on a computer and it requires that unique id.  the data is actually retrieved directly from apple’s servers and may differ in version from the software on your devise.  you can even cache your blobs on saurik’s server right form umbrella without ever violating your apple warrantee or modifying your devise in any way.

overview of the process and reasons:
http://www.saurik.com/id/12

official umbrella site:
http://thefirmwareumbrella.blogspot.com/