If an upgrade is scheduled, the network administrators will receive an email notification approximately one week prior to an upgrade. Administrators can choose to reschedule the upgrade for up to one month, or perform it manually via the Dashboard on the same page where the firmware upgrade window is configured. If no further action is taken, the upgrade will be scheduled within the firmware upgrade window specified by the network administrator(s).
https://documentation.meraki.com/General_Administration/Firmware_Upgrades/Cisco_Meraki_Firmware_FAQ
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