4 K2MnVIO4 + 2 H2O KMnIIIO2 + 3 KMnVIIO4 + 4 KOH

This is an oxidation-reduction (redox) reaction:

3 MnVI - 3 e- 3 MnVII (oxidation)

MnVI + 3 e- MnIII (reduction)

K2MnO4 is both an oxidizing and a reducing agent (disproportionation).