This article describe a way to upgrade SSR (WAN Edge) device from Mist portal.

WAN Edge devices connected to Mist can be upgraded from the cloud. To schedule a device to download and upgrade to a specified version

Below are the steps to upgrade SSR (WAN Edge) devices via the MIST Platform:

1. Go to WAN Edges and select the Site from the UI.

2. Click on the SSR device that needs upgrading.

3. Once the WAN Edge Page is available, select Utilities > Upgrade Firmware.

4. Choose the upgrade version from the drop-down menu.

Note: 

Beta - contains all the Firmware versions ( including the latest ones which are under testing ) 

Production - Only the stable Firmware versions

As the download and upgrade operation is sent to the device, you can monitor progress by visiting the WAN Edges page for the site.

To see detailed events about the operation, visit the WAN Edge Events in the device insights page.

For the SSR device in HA (High Availability):

For WAN Edge devices in a High Availability (HA) cluster, minimize downtime for upgrades by selecting one node in the cluster first.

After you perform the upgrade operation on that node, move to the next device and repeat the process.

https://www.mist.com/documentation/upgrading-wan-edge-ssr/