Download the required firmware on Legrand developer portal

You can check which firmware is required for each Legrand devices in the Compatible products section. A firmware is a .zigbee file

You must be logged in with your developer account to access the firmware download page

Upload the firmware on the gateway of the ZigBee network

The method depends on the gateway you are using :  depending on your case, you may do it manually or use a release management tool. Please refer to the update management process of your gateway

Trigger the update of the ZigBee device

The process to trigger an update for a ZigBee 3.0 device is described in the ZigBee documentation. Legrand modules automatically send update requests to the gateway