In general, when the new firmware upgrade is available for your network, then it will be available under Networkwide > General, once the firmware will be available then you will be able to see following three options (default behavior),
And if the firmware is not available, then it will be greyed out,
You can also manage the firmware upgrade on the organization level by scheduling the firmware for the particular network under Organization > Firmware upgrade > All networks > Schedule upgrades.
I would highly recommend you to refer below documents for all the firmware related queries,
https://documentation.meraki.com/Special:Search?q=firmware&path=&limit=11&page=1&tags=