Core board is the middle board that is at the center of the brain box.
1. Remove the 3 screws at each corner on the core board.
2. After that, lift the board from the top end edge in the picture and wiggle until it comes off.
3. Locate two ribbon cables on the right hand side of the board. Use a small knife to lift up the black tap and remove them. Top ribbon cable has the blue end facing up. The bottom one has the silver end facing up.
The bottom ribbon cable.
4. Use a plier to remove the silver screw on the GPS antenna.
After that the core board will come completely detached.
Repeat the steps for the other core board.
Choose the NEW core board and install it onto the OLD brain box. Now you do not need to swap out the black display control circuit board mentioned before.
When you install the core board, Install the two ribbon cables first before installing the brass antenna.
Of the two ribbon cables, the shorter one goes to the bottom of the core board and the longer one goes on the top.
Remember the shorter one has silver end facing up.