Hi,
I’ll try to provide a general perspective on the matter.
There are several ways ROOT can be installed on ubuntu 18.04, all listed at https://root.cern/install as @couet suggested. What’s the method to upgrade an existing installation depends on the method you installed ROOT with in the first place.
For example, if ROOT installed via a package manager such as conda or apt (e.g. using @amadio’s .deb package) upgrading is just a matter of telling the package manager to install the newer version – that should also remove the old one as the package manager knows it’s a conflict. At worst, you’ll have to tell the package manager to uninstall ROOT and then install the newer version, which is maybe two commands instead of one. Still manageable.
Compiling ROOT from source like you did is the best way to customize ROOT, but then you have the responsibility to manage that ROOT installation. If you have not installed ROOT in system directories such as /usr/local after compilation (you can check where your ROOT installation resides with which root, for example), then upgrading should be a matter of removing your current installation (e.g. removing the build/install directory) and compiling again. If you did install ROOT in system directories, you have to manually remove it from there.
As using a package manager takes care of a lot of that complexity, I would suggest installing ROOT as a conda package or a .deb package. If you need special compilation options or tweaks, then you can compile ROOT yourself, and manage where ROOT is installed as well as removals manually.
Also: we are working on an official .deb package based on @amadio’s work, so soon we will have a one-liner apt invocation that installs ROOT and makes it easy to manage the same way as you manage any other package on Ubuntu or Debian. Work in progress 
Hope this clarifies things a bit!
Cheers,
Enrico