Login to Webmin and configure Webmin.
Select System then Software Package Updates then Scheduled checking options. Select the following options, enter your email address, then [Save].
Check for updates on schedule? Yes, every day.
Email updates report to: firstname.lastname@example.org
Action when update needed: notify
There is a lot more work to do. The total work depends on what you need. Apache? MySQL? An email server? Do you want FTP with that?
Looking through the rest of Webmin is easier when you have Webmin running. Create a test Webmin somewhere. You can use the slowest oldest spare computer from your junk bin. For a backup system, your main requirements are reliability and big new disks, any disks older that three years are going to fail soon. A Web server will require plenty of memory and should have a similar amount to your VPS, or dedicated server, so you can test similar workloads. Definitely use the same operating system and application versions as your main Web sites to pick up any compatibility issues.
After using Webmin for a few months, you might decide to throw out cPanel to save a few dollars on your Web site hosting cost or you might decide it is all too hard and stick to cPanel. At least you make the decision from experience, not from second hand reports on Web pages from people who are not performing the same work as you.