I have LibreOffice installed from the repositories (currently it's version 6.0.7 in 18.04.1) but it's possible to download version 6.1.4 from their website as a .deb package. If I wanted to install this newer version over the old, would I first have to remove version 6.0.7 with an apt-get remove, or could I just install the new .deb package and things will be taken care of automatically?
Uninstall the existing LibreOffice apt package from the default Ubuntu repositories first before you install the new LibreOffice .deb file or you will end up with two instances of LibreOffice installed alongside each other. The same thing goes for installing the LibreOffice snap package alongside the LibreOffice apt package. You can have both versions of LibreOffice installed alongside each other and use either one.
The LibreOffice snap package (libreoffice) has the advantage over a downloaded LibreOffice .deb file that it is updated automatically when updates become available.