MM is the actual miner firmware.
OpenWrT is the miner controller software.
PMU is for the power managment unit (where the PCIe plugs are)

Just make sure the MM and OpenWrT are from the same release dates, they are matched pairs.

For OpenWrT open GUI and go to System > Backup/flash firmware to start things. By doing the upgrade from here you have the option to retain your pool & users settings.
For the MM, after doing the OpenWrT and rebooting controller, open GUI and go to Status > MM upgrade and againdo the obvious.