Webmin v1.780 is available for TurnKey v14.x and a quick glance over the changelog for Webmin v1.791 suggests that there are not any significant changes unless you wish to use Webmin for Let's Encrypt SSL certs. Although perhaps I missed something?
Is there something specific that you were missing or that was changed in the later version? Or did you explicitly want the Let's Encrypt stuff? Or were you using an older version of TurnKey?
Also be aware, that when you next run apt-get upgrade it will most likely reinstall the old version of Webmin again. So you'll need to remember to go back into Webmin and manually "Upgrade Webmin" every time you run apt-get upgrade...
What version of TurnKey are you on?
Is there something specific that you were missing or that was changed in the later version? Or did you explicitly want the Let's Encrypt stuff? Or were you using an older version of TurnKey?
Also be aware, that when you next run apt-get upgrade it will most likely reinstall the old version of Webmin again. So you'll need to remember to go back into Webmin and manually "Upgrade Webmin" every time you run apt-get upgrade...