i would like to update to krypton for my mxq pro box

heel this is my first time posting here so im not entirely sure where i should post this i own three android boxes and all of them have kodi and are in need of an update to krypton i have tried to download the update but they all say the app cannot be installed and it is on android version 5.1.1 please help me install the update all help will be appreciated

Thank you,