XG Firewall Firmware Upgrade Failed: 17.5.16 MR-16 to SFOS 18.5.3 MR-3-Build408

Hello Ankit Na,

Thank you for reaching out to the community, when the upgrade fails is it going into failsafe mode?

Please refer the following KBAs below: 
> Reference points when upgrading to SFOS v18: https://support.sophos.com/support/s/article/KB-000038897?language=en_US
> Know the cause of hardware appliance going in failsafe mode: https://support.sophos.com/support/s/article/KB-000036375?language=en_US

Also log a support case, if it requires any further investigation.