No matter how hard core cli user you are it very often that you will need a GUI to perform some tasks just because, you can’t remember the commands or you want a better visualization (Firewall ports comes to mind).
SSH to your server, get root privileges and update the package indexes.
sudo su apt update
In order for your installation to trust the Webmin repository you need to install the PGP key and convert it to a format that apt can use to verify the files
curl -fsSL https://download.webmin.com/jcameron-key.asc | sudo gpg --dearmor -o /usr/share/keyrings/webmin.gpg
Edit the repositories list
Scroll all the way to the end of the file and add
deb [signed-by=/usr/share/keyrings/webmin.gpg] http://download.webmin.com/download/repository sarge contrib
Save and Exit
Update the repositories and install webmin
apt update apt install webmin
Now you can access the GUI interface by
https://<IP address or FQDN>:10000