Was no need to uninstall 5.
You installed 6 from here?
You should be able to just type the three lines of command into your terminal and it just happens.
According to this: [Solved] Mint 18.2 will be based on which version of Ubuntu ? - Linux Mint Forums Linuxmint 18.2 is based on Ubuntu 16.04 which is not supported by the KiCad PPA. (Xenial is listed in the dropdown but the oldest Ubuntu packages in the repo are for 18.04. I guess the webpage needs updating.) Both Ubuntu 16.04 and Linuxmint 18.2 are out of support too.
Sorry, you’ll have to upgrade your distro. That Ubuntu release has been around 6 years after all.
Sorry, missed the 18.2.
Kicad installs happily on mint 20 & 21 by just typing in the three above mentioned commands into your terminal.
I was using mint 20 but have just upgraded to 21.