Navigate to the Template tab and pull up the Device Group in question. Then, go to Setup > Session > MTU and configure a value.
Perform a local Panorama commit
Perform a Template commit
Note: If a template value is pushed when there is no MTU value configured on the device (value is set to blank or left as the default value), it will be reflected on the device. If the managed device has a MTU value configured, then the template value will not be displayed. Instead, the value configured on the device will be displayed.