OK, so if you downloaded a .deb file to your Downloads folder then open a terminal and do
cd ~/Downloads
sudo dpkg -i packageFileName.deb
where packageFileName is whatever the name of the file is.
Bluetooth on the Librem 14 is unfortunately a learning curve. Probably best to continue in the topic that I linked to: Librem 14 - no bluetooth
A word of caution: This is not the normal right way to install software in Linux. It is needed in this specific circumstance because Purism does not provide support for Bluetooth in its repository for any laptop. For anything that is in the PureOS repository, you should get it from there and install using the normal approach e.g. using the PureOS Store GUI application.