How I can reset my hero exp because this hero is not usable?

You can split heroes that you do not want to keep and this will return some of the wisdom medals and money spent on upgrades but you do not get back the EXP points.

To split the hero go to the hero menu and tap on the hero to open their screen. Then tap on the small book icon in the op right to view their 'story'. If the hero is not being used you will see an icon in the lower left to split it. Your hero will be removed.