How do I reset a run on coolmathgames?

you can clear your browser cache. If you don't know how to do that, there's many ways to do it, by googling you should be able to find a way that works for you.

just so it stays here
all you need to do is open up settings (clicking the 3 dots in the top-right and clicking on settings)
and go to "Cookies and other site data"
from there scroll down and click on one of the three "Add" buttons (preferably one of the last two)
now just paste the link you use to play one of the games and now you're ready
so whenever you need to reset the game data, just click on that little arrow that faces towards the right and click "Clear data"

Old post but that could be hella tedious…