Ubuntu 18.04: How to update virtualmin 5.04 to 6.06-2? [RESOLVED]

I have two servers, both on Webmin 1.910. The newest, set up 2 months ago, has all the settings in the repos for apt-get to just update everything each time. The oldest, from 3 years ago, had virtualmin installed with the install script. Somehow it’s gotten stale over the years. Ideally I’d like the old one to behave like the new one, but adding the lines to /etc/apt/sources.list taken from the new one resulted in

Err:6 http://software.virtualmin.com/vm/6/gpl/apt virtualmin-bionic InRelease
The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY D9F9010760D62A6B
Err:7 http://software.virtualmin.com/vm/6/gpl/apt virtualmin-universal InRelease
The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY D9F9010760D62A6B



The lines in the one which works are:

deb http://software.virtualmin.com/vm/6/gpl/apt virtualmin-bionic main
deb http://software.virtualmin.com/vm/6/gpl/apt virtualmin-universal main



But I’m unsure i should just be sticking them in to the old one like this. So I thought I could just run the install script again, but when I tried it told me in no uncertain terms it would break the installation:

But, if Virtualmin has already successfully installed you should not run this script again! It will cause breakage to your existing configuration.
So I'm looking for a way to get the old server on the same update system as the new one.

Thanks!

[Update]

On the newer system which works:

dpkg -l webmin-virtual-server

Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Architecture Description
++±============================================-===========================-===========================-=============================================================================================
ii webmin-virtual-server 6.06.gpl-2 all Webmin module for ‘Virtualmin Virtual Servers’



On the older one which doesn’t:

dpkg -l webmin-virtual-server

Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Architecture Description
++±============================================-===========================-===========================-=============================================================================================
ii webmin-virtual-server 5.04.gpl all Webmin module for ‘Virtualmin Virtual Servers (GPL)’


[RESOLVED] Fixed by adding the key as described here, and using the manual edit to sources.list as described. https://www.virtualmin.com/node/53076