Webmin is open source control panel for linux machine, it is mainly use for server as file transfer, email configuration and other servers configuration using modern user interface.

Here is the step to install the Webmin on ubuntu

1) Open the command prompt of your server and switch to root user

2) Now add webmin repository to source list file using command or any editor

vi /etc/apt/sources.list

and add this line at bottom of file 

deb http://download.webmin.com/download/repository sarge contrib

Now save the file

3) Now download and add PGP key for webmin using following commands

wget http://www.webmin.com/jcameron-key.asc
sudo apt-key add jcameron-key.asc

4) Update list of package of update using

sudo apt update

5) Install webmin using 

sudo apt install webmin

If no error on screen, then you will install webmin. Now open webmin using browser as instructed using 

https://your_server_ip:10000

and login with your root password.