No, your devices will be automatically upgraded at some point in time:
"Firmware upgrade are deployed periodically by Cisco Meraki to all customers in a series of batches. These periodic updates are designed to maximize the availability of new features on the most stable firmware available, while minimizing the amount of downtime required during the year to perform the upgrades. At least a week prior to the periodic upgrade, all administrators configured to receive network alerts will receive an email informing them of the change and a notice of the upgrade will also be posted in Dashboard."
https://documentation.meraki.com/General_Administration/Firmware_Upgrades/Cisco_Meraki_Firmware_FAQ#...
There is a way to reschedule or cancel an automatic upgrade though:
"Note: Firmware upgrades can be scheduled up to one month in advance and can only be deferred/rescheduled for one month at a time. Cancelling scheduled firmware upgrades does not exclude a given network from future scheduled upgrades."
https://documentation.meraki.com/General_Administration/Firmware_Upgrades/Managing_Firmware_Upgrades...