If you need the disk space then un-install the old version once you are sure you no longer need them.
If disk space is not a problem then there is no need to un-install at all.
I have every version from 15, plus various builds for some versions and also a couple of different national version.
They all work fine and don't interfere with each other.
If you like to double click on a PLN file to open it, generally the last version installed will be the default software.
But you can set in your operating system which program will be used to open a PLN file by default.
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10