Not really. If you are getting certificate errors then the problem is not something that you are doing but rather something that the server is doing.
You may need to follow: Just installed PureOS9 - Error certificate NOT trusted - what do i do with this error? - #29 by irvinewade (instructions courtesy of @amarok).
Once you have applied all updates, it will subsequently accept the needed certificates and you can revert the change - and you should be good to go, at least for a few years until the next certificate expiry.