What’s your issue? The MXs should revert to their previous configuration after a config change if it loses connectivity to the Meraki cloud. I believe the same goes with a firmware upgrade.
There is also the reset button on the MXs too. It forces the unit to clear its configuration and fetch it from the cloud again. So you can correct the configuration in the Dashboard and reset the device, so it grabs the new configuration.
EDIT: if you’re just talking about a restart there is always the power cable.