How to install MATE and remove KDE

> How can I install MATE so I can try it instead?

$ sudo apt install trisquel

> If I then decide to use MATE, I won't need KDE anymore. So how would I
> uninstall KDE?

At that point you might as well reinstall Trisquel using the regular
ISO. If for some reason this is not an option, you could try running

$ sudo apt remove triskel
$ sudo apt remove plasma-*
$ sudo apt autoremove

This might not remove every trace of KDE, though.