Once the earbud is open, you will see two tiny circuit boards linked by a board-to-board connector.

Using an Opening Pick, unplug the two circuit boards by levering very gently between them.

Caution: Do not use a metal tool for this step, as the battery is still inside the earbud and you may unintentionally short-circuit components as they are still powered.

Set the upper part of the earbud shell aside in order to reassemble it later.

The upper part is highlighted in red in the third picture.

The lower part is highlighted in dark blue in the third picture.