The Power of UN — Nancy Etchemendy

Until a week ago, Gib Finney was just a regular guy, shooting spitballs and messing up his science experiments. But when he finds a mysterious device called the Unnder, everything changes. Gib discovers that the Unner has the power to erase any and all mistakes. At first, Gib thinks this is better than winning the lottery: he'll never flunk another test or strike out again!

But after a terrible accident, Gib must decide which events of the previous day he must undo to stop the disaster from happening the second and third time around. He soon learns that some things are more worthy of 'unning' than others, and some things can't be changed at all.