1. Turn on the stereo.
2. Press the "Menu" button on the stereo.
3. Use the arrow keys on the stereo to navigate to the "Clock" menu.
4. Press the "OK" button on the stereo to enter the "Clock" menu.
5. Use the arrow keys on the stereo to set the hour and minutes.
6. Press the "OK" button on the stereo to save the time.
Note: The steps may vary slightly depending on the specific aftermarket stereo model. Refer to the stereo's user manual for more detailed instructions.