From the NetScaler management console:
Log on to the NetScaler management console.
Navigate to System > Backup and Restore.
Highlight the file which you want to download.
In the Action drop-down menu, select Download to download the file.
You can also right-click on the backup file and choose Download.
From WinSCP:
SSH to NetScaler.
Using the right-hand pane, navigate to /var/ns_sys_backup folder which contains the backups.
Highlight the file that you wish to download and select Download from above the pane.
You can also right-click the file that you wish to download and choose Download from that menu.
Citrix Documentation - Backing up and Restoring the NetScaler Appliance