Hi
Probably it is possible to upgrade your stack by GUI, but i always work with CLI so i can tell you how to do it that way :)
If you are using a tftp server for retrieving your firmware, enter following command on the commander switch:
copy tftp flash <ipaddress tftp> <filename> primary
The commander will push this automaticly to the other members of the stack.
When this is complete you will need to boot the whole stack with following command:
boot system
Kind regards