To upgrade the Edge SWG or ASG software:

1. Download the .bcsi SGOS file from support.symantec.com > Downloads > Network Protection Downloads
2. Select the Maintenance Tab > Upgrade > Upgrade Tab
3. Click Upload
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4. On new Browser Tab that opened, Browse for the image file > then Click Install
5. The Browser Tab will spin until complete
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6. Once Complete the page will update to indicate No errors and No Warnings
7. Then Close the Browser install image Tab
8. From the Maintenance Tab > Upgrade > Upgrade Tab > Click Reboot the default system (newly uploaded images are Default automatically)
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9. The new System Image with then boot up after restart

Systems Tab Information:

In the Edge SWG or ASG Management Console under Maintenance Tab > Upgrade > Systems tab, it displays the five available systems. Empty systems are indicated by the word Empty.
The system currently running is highlighted in blue and cannot be replaced or deleted.
From this screen, you can:

• View details of the available SGOS system versions.
• Select the SGOS system version to boot from (Default System)
• Select the SGOS system version to be replaced
• 

To view details for an SGOS system version:

1. Select the Maintenance > Upgrade > Systems tab.
2. Click Details next to the system for which you want to view detailed information; click OK when you are finished.

Setting the Default Boot System

This setting allows you to select the system to be booted on the next software/hardware restart. When a new system is uploaded, it is set as the default boot system automatically. If a
system fails to boot, the next most recent system that booted successfully becomes the default boot system.

1. Select the Maintenance > Upgrade > Systems tab.
2. Select the preferred System version in the Default column
3. Click Apply.

Locking and Unlocking Systems

Any system can be locked, except a system that has been selected for replacement. If all systems, or all systems except the current system are locked, the Edge SWG appliance cannot upload a new system. If a system is locked, it cannot be replaced or deleted unless by CLI with verification of delete and unlock(below)

Locking a System:

1. Select the Maintenance > Upgrade > Systems tab.
2. Select the system(s) to lock in the Lock column.
3. Click Apply.

To unlock a system:

1. Select the Maintenance > Upgrade > Systems tab.
2. Deselect the system(s) to unlock in the Lock column.
3. Click Apply.

Replacing a System

You can specify the system to be replaced when a new system is downloaded. If no system is specified, the oldest unlocked system is replaced by default. You cannot specify a locked system for replacement.

Specify the system to replace:

1. Select the Maintenance > Upgrade > Systems tab.
2. Select the system to replace in the Replace column.
3. Click Apply.

Deleting a System

You can delete any of the system versions except the current running system. A locked system must be unlocked before it can be deleted. If the system you want to delete is the default boot system, you need to select a new default boot system before the system can be deleted. You cannot delete a system version through the Management Console; you must
use the CLI:

How to Remove an SGOS Image from the Edge SWG (ProxySG) or Advanced Secure Gateway

Also See how to upgrade VIA CLI if GUI Management Console is not working:

Upgrade Edge SWG (ProxySG) or Advance Secure Gateway via command line interface