For the upgrade of SSR devices from MIST UI, please provide the steps on how to do it and what are the pre-checks.

N.A

Before considering upgrade of SSRs and it is recommended to go through the release notes for details. For example, here is the release notes for 6.2 versions  SSR 6.2 Release Notes | SSN Docs (https://www.juniper.net/documentation/us/en/software/session-smart-router/docs/release_notes_128t_6.2/)

Here is procedure for SSR upgrade from MIST. Upgrading WAN Edge SSR - Mist (https://www.mist.com/documentation/upgrading-wan-edge-ssr/)

Pre-checks:

Make sure the device is in Connected state. 

Make sure there are no critical CPU or Memory issues shown in "WAN Edge Insights". 

Monitor and make sure there are no critical events shown in "WAN Edge Insights"