Upgrade 3.0 OS to 4.0

[gosat13]

My iPhone is a 3GS iOS 3.0 jailbroken and unlocked for T-Mobile SIM
card by the vendor.
Now is time to upgrade in order to use updated apps that don't work with 3.0.
I have been told by the vendor that i can upgrade to 4.0 and then jailbrake it,but after that will it work with
T-Mobile ?
I would like to have good advise on how to do it with detailed procedure,because i never fooled around with.
Is anyone outthere willing to help me ? will appreciate.
Thank you

[Simon]

Do you have access to a Mac?

[iYeow]

Make sure you have itunes atleast 9.2 to 10
Now download Snowbreeze 2.0.2 > Use Expert mode > General > Uncheck activate my iphone.
Build it and it will be named Snowbreeze_4.1.ipsw and placed on your desktop
Having your iphone plug in and the official sim card inserted.
Put your jailbroken iphone at 3.0 into Recovery mode
Now open up Itunes, you are in restore mode > do the Shift + Restore to Snowbreeze_4.1.ipsw.
If successfully restored, you are already jailbroken at 4.1.
Unlock with Ultrasn0w from Cydia.
Now your iphone will be activated and jailbroken at 4.1
Setup as a new iphone.

If you have access to a Mac, then restore to custom firmware created by Pwnage tools 4.1.2 : After preparing the custom firmware, you will be asked to put your iphone into DFU mode and then do the Shift + Restore to Custom firmware of 4.1

[gosat13]

Do you have access to a Mac?

Sorry only Windows 7 64 bit.
Thank you

[Simon]

Just follow Yeow's advice and you should be fine then.