The steps to replace the battery of SG4 audio are as follows:

1. Turn off the car audio first, and unplug all interfaces and power cords.

2. Use a Phillips screwdriver to unscrew the screws at the bottom, remove the battery compartment cover, and take out the original battery.

3. Place the new battery into the battery compartment, paying attention to the alignment of the positive and negative poles, cover the battery compartment cover, and tighten the screws to secure it.

4. Connect the power cord and other interfaces, and start the test.