Why can I not downgrade my firmware 9.3.4 to 9.3.3

[iMore Question]

I have an iPhone 6 Plus with tmobile. I updated my phone to the 9.3.4 and I want to go back down to the 9.3.3. I downloaded the correct IPSW, but apple keeps giving me the firmware file is incompatible. I updated my PC and made sure iTunes was updated as well. I need help

 

[JML5150]

You would have to put your phone in DFU mode, If you didn't already. The perform a shift/command restore. Also check that you have the GSM version of 9.3.3. for your device