Manually downgrading virtualmin to GPL (free version) & updating of etc/apt/sources.list.d

sudo su

nano /etc/virtualmin-license
#add the below lines, set values to GPL
SerialNumber=GPL
LicenseKey=GPL

#command derieved from from virtualmin installaion script
wget https://software.virtualmin.com/lib/RPM-GPG-KEY-virtualmin-7
gpg --import RPM-GPG-KEY-virtualmin-7 && cat RPM-GPG-KEY-virtualmin-7 | gpg --dearmor > /usr/share/keyrings/ubuntu-virtualmin-7.gpg

sudo nano /etc/apt/sources.list.d/virtualmin.list

#manually replace V6 with V7
deb [signed-by=/usr/share/keyrings/ubuntu-virtualmin-7.gpg] https://software.virtualmin.com/vm/7/gpl/apt virtualmin main

apt update
apt upgrade