In order to reset the OBC you need a paper clip or a pen. Stick the paper clip in to the small hole next to the number one in the bottom right corner. Press the appropriate buttons along the bottom to change the time. 1000, 100, 10, 1
You need to press the 1000 button to go from am to pm.
Pressing the AVG MPG button will change from metric to US gal.
Press set-res when done.
FYI - if you light is out, there is a light bar on the left side of the unit. You will need to completely remove the OBC to change this. I believe it's about $35-45 for a new light bar.