Linux MX is a Debian based distribution, therefore the packages are often compatible.
But, KiCad 6 is available only through the backports package list. To install KiCad 6 we need to update the apt list and install the correct package.
$ sudo nano /etc/apt/sources.list
Then add the following lines to the file
deb http://deb.debian.org/debian bullseye-backports main contrib non-free
deb-src http://deb.debian.org/debian bullseye-backports main contrib non-free
Then we can install the backports specific version like so
$ sudo apt install -t bullseye-backports kicad