Is there a way to undo a lap if I press the lap button by mistake during a run activity?

No, not during the activity.

See: how-to-correct-push-of-lap-button-during-a-workout

I already submitted the idea via https://www.garmin.com/en-US/forms/ideas.

Others have mentioned you can correct it in the file, with fit file tools.

Garmin could really do with implementing a ‘back’ or essentially an ‘undo’ here. It’s a bit silly that you can accidentally skip forward in a workout by pressing lap button, but have no way to go backwards - potentially completely ruining a workout midway through. 
I guess it’s also annoying if you pressed it but didn’t really want to record a lap but I see that as a minor issue compared to workouts. 

I did this today. Luckily I'm a slow out of shape noob so I have bigger problems, but it's still so annoying you can't fix it.

But one of the people in the other thread linked above had the best band-aid fix in the meantime. Go to the activity, settings, disable lap button. 

Then if you still want to have the option of using the lap button (on purpose, not by accident) set up a hotkey such as "hold back button". Much less likely to accidentally hold the button down for a length of time. I'm happy with this solution.