Vitux published a tutorial about how to change Debian's default applications.
How to Change Debian’s Default Applications
We all have our preferences when it comes to the application we want to use for opening and working with a certain file type. For example, whenever I start using a new version of an operating system, I install and switch to the VLC media player for playing music and videos. In Debian, you can change your default applications both through the command line and the graphical user interface through the simple steps described in this tutorial. We have performed the commands and procedure described in this tutorial on the latest Debian 10 Buster system.
