https://en.wikipedia.org/wiki/Manganese_dioxide#Reduction

I think the easiest way to convert manganic into manganous form is to keep  the salt  in excess of water  for few hours , probably , you will start seeing the change in color of the solution .  In a soil system , we achieve it like this only .   

According to Holleman-Wiberg`s Inorganic Chemistry, if you heat MnO2 above 1172°C you`ll get MnO:

MnO2 -> MnO + 0.5 O2