The transfer belt is the T508, pictured here:
The transfer belt in this machine is located just behind the front door (that swings out and downward), and attached to it / mounted on it.
In front of the toner cartridges, in short, as positioned when the printer is closed up and working.
Here's how to remove the old transfer belt:
1. using the handle, release the catch on the front door, and swiing it forward (towards you) and downward.
2. then press the green release handle to release the transfer belt, and lift it up and out of the machine... note: do not touch the belt surface itself, it is quite sensitive. Likewise do not clean it, do not use liquids of any kind on it... This especially applies to the new transfer belt.
To put in the new transfer belt, aside from observing greater caution in handling it, and also having to unpack it, you just do the reverse of that.
You may also have to do a Transfer Belt Reset to clear the transfer belt message... and also a fuser reset no doubt. Let me know if that proves needed, and I will promptly get you the information on how to go about those two resets.