The tender top is secured with a latch under the coal shute. Press it in, far enough to relelase it but not too far, you don't want it to break. I find that tucking thumb and finger nail under the bottom of the green tender top is enough to apply a little upward pressure which enables me to feel when the latch has released. Lift up the front, and the rear will be able to escape its two similar latches.

Note the hole under the latch where the +ve and -ve pickup wires emerge to the loco. With the DCC chip in the tender, we will need five wires to come through that hole. A little modification will be necessary.