Swapping out an internal part like a PCB is a simple process, but should be done carefully to not damage other components in your TRMNL.
First, turn off the device. Then open it up (video guide), laying it flat like this:
Removing the PCB
Next, use a small Phillips screwdriver to remove the 3x screws on your PCB. A flat-head also works, but you'll need to be careful to avoid stripping.
After removing all 3 screws, slowly lift the PCB board and detach the battery by pulling straight outward on both wires (red/black).
Changing the battery
If you are changing your battery, be mindful that there is adhesive holding it in place, so carefully pry it up slowly using a flat, rounded-edge tool (puncturing the battery is hazardous!).
Then, continue to reattach the wires once replaced and rescrew the PCB to the case.
Disconnecting the PCB and the Screen
We're almost done. Use a fingernail or screwdriver to gently "open" the black gate, located along the back edge of where the ribbon cable attaches to your board. The gate, when closed, is parallel to the board.
This will allow the ribbon to slide out of the board with almost no effort. Notice that the open gate is now perpendicular to the board.
Here's another angle.
You may now replace the board or the screen.
Replacing the Screen
If replacing the screen (EPD), you want to flex the case out of the way, not the screen, to remove it from its held position. The EPD should never have pressure placed on it at a single point, or it will cause it to break. Imagine a cupcake or loaf of bread in a pan; you don't want to squish it, so you adjust the container (the case), not the object (screen).
Reattaching the Screen to the PCB
When reattaching the screen to the new or old PCB, you need to insert the ribbon cable again with the black gate open (perpendicular). When inserting the ribbon cable into your new PCB, be sure not to insert it diagonally.
Here's another example, with the board inserted at a diagonal angle. This will not function.
Insert the ribbon cable straight, but note that the gold section of the ribbon cable will not be completely covered by the white/black connector.
Seeing a narrow, even-width section of exposed gold pins is the expected installation. You can now lower the black gate to lock the cable in place.
Reattaching the Battery
Lower the board back into place, re-attaching the battery. The red wire should be on the side of the USB-C cable.
Finally, add back the 3x screws. We recommend starting with this one first (farthest from battery connector), as it will help "seat" the PCB into the tight tolerances of the enclosure.
Close your case (video guide) to complete this procedure. Turn the device back on, then re-pair WiFi and add the device to your account as if it were brand new.