Gameboy cartridges cannot be reset unless there is a setting on them, generally to erase all saved data.
However, if you remove the battery inside the cartridge or allow it to die, this will erase the data as well.
There are several ways to do this. First you may want to go to the main screen of whatever game you wish to reset the stats and scores of and under options it may say something like new game or reset data! If that doesn't work you can always try a magnet to the cartridge I don't recommend this though.