You may need to change out the rear PU sleeve on your G2 Black at some point or check the motors. Here is a video showing you how to swap out the PU sleeve.
Replacing a battery may seem like a stressful process but it is very simple. Again please make sure everything is turned off and your hands are dry before beginning this process. The video below will show you how to safely swap your battery. If your replacement battery comes in a case already just follow the process of replacing the connectors from the board to the battery. (The video below is a old model G2 in 2017, but the process remains the same)
For more general electric skateboard maintenance check out Daniel Kwan's video for how to take care of your board. Remember to regularly inspect all screws on your board and tighten them as necessary.