I think you are referring to the old 2.5 user guide. Please see page 28 of the latest 2.6 version here:
https://www.vmware.com/pdf/app-volumes-26-users-guide.pdf
Yes, you can update the existing AppStack. Please follow the below steps:
1. Click on 'Volumes' -> 'AppStacks'. Select the AppStack and click 'Update'
2. A copy of the existing AppStack is created. Provision the new AppStack to a VM for updating
3. Update the applications and reboot VM
4. Once the update is complete, the new AppStack is created with the name "AppStack-update"
5. Unassign the existing AppStack
6. Assign the new AppStack