If it is a modern-era Stern, you can easily find Grand Champion as you go through the Adjustments or Features Menu. It's in one of them, but can't remember which. I see it all the time when I make modifications to my pins. The rest of the high scores are right in that same spot too. Most other machine brands have similar adjustments programs and you should be able to easily find the high scores (usually 4 levels).
I have reset my scores many times to give visitors a chance to experience the fun of putting up their initials. BUT, I ALWAYS leave the Grand Champion score alone. Like a couple of others on this post have already said (and I whole-heartedly agree with!), the best way to do this is always leave the Grand Champion alone but reset all the others. And I leave my Grand Champion score on the original Factory Default. I figure THEY knew what was a decent score to set it at!!!