If you have a jailbroken and unlocked iphone. Or possibly NOT currently jailbroken, but you NEED an UNLOCK and want to UPGRADE. Or if you just want to upgrade but retain your current baseband for future unlock.......FOLLOW THIS>>>

Tiny Umbrella
Sn0wBreeze (for Windows User's)
PwnageTool (Mac User's)
Firmware's / ipsw

FIRSTLY and most IMPORTANTLY : Have you downloaded Tiny Umbrella? Well Do it!!
Have you hooked up your phone to computer and pressed the "Save SHSH" button? Well do it!!

Secondly : Have you Downloaded the correct ipsw to which you want to upgrade?.........DO IT!!

Thirdly : Got a Mac? Well download Pwnage Tool.
Using Windows?? Get Sn0wBreeze

Now, This tutorial is for upgrading to 4.2.1. For upgrading to LOWER firmware's, it's the same process.....But earlier version's of these tool's. After you have your shsh's saved and the appropriate file's downloaded......proceed. (For this i will be using Sn0wBreeze, but Pwnage is VERY MUCH similar)

HERE GOES!!!!!!!!!!!!!!!!!!!!!!.........................................

Step 1 - Open Sn0wBreeze and press ok (unless you actually DID pay for the software??)

Step 2 - After closing the credit's popup, press the BLUE arrow, only after you have your file's and know where they are.

Step 3 - Either Drag your downloaded ipsw file, or browse your computer for it. Your choice.

Step 4 - Once the ipsw has been verified, click on the BLUE arrow again.

Step 5 - This is quite important. It give's you 3 choice's.
1 - Baseband Preservstion only (Will not jailbreak device, will only create a custom ipsw, that upgrade's and save's your baseband. Usefull for those that want the Greenpois0n animated BOOT LOGO on all device's)
2 - Simple mode Will install cydia and not much else. For those that want to add them manually through CYDIA.
3 - Expert mode. For those hardcore user's who KNOW what they want. Install package's, make partition's ect.

Step 6 - Click the BLUE arrow once again, to begin creating your CUSTOM IPSW. (While it's creating, it give's you the option to take a break from this nervewracking experience, and enjoy a calm game of pacman.....untill the ghost's get ya!!.

Step 7 - Once it has finished creating, You have the option to put your phone in Pwned DFU mode (required for custom restore's) Just follow the onscreenj instruction's. Simple!

Step 8 - Once in Pwned DFU mode, it will give you a popup TELLING YOU, you can now restore in itunes.

Step 9 - Open itunes, Press Shift+ click the RESTORE button. And select the CUSTOM IPSW you just created, and press ok.

Step 10 - All should be well. If you get an error 2013, then restart computer and try again.
For any other error's, try the following>>>>>
- Different USB Port.
- Different Computer
- Different OS (try using a MAC if it's possible)
- Different Sync Cable.
- Try running With / Without Tiny Umbrella tss server running in Background.

STEP 11!!!!! Follow THIS to get your phone unlocked again, and some source's for more stuff from CYDIA.