I have just installed Buster using 2019-06-20-raspbian-buster-full.zip.
When I try to update from the menu I get an error saying a package could not be retrieved.
When I use 'apt-get update' I get an error message saying 'testing' has been renamed 'stable'.
It also says explicit approval is needed to use the repo, but no indication how, other than refer me to
'man apt-secure' which I did not find useful.
How do I update Buster?