As I would like a French Dictionary extension (French Dictionary "Classic+Reform 1990), provided by Dictolecte that is different from the version already installed after the download and installation of LibreOffice, and as you can’t use both of these Dictionaries at the same time, I have to delete the first one to be able to use the latter.
There’s a LO forums thread describing how to deinstall those “default” extensions on Linux and Windows. Hope this helps.
I like (and I use sometimes) this quick-and-dirty solution
Assuming this is under Windows, just go to Control Panel, Add or Remove Programs, select LibreOffice and press the Change button to change your installation and then choose Modify. This will show a list of all the options. Go to Optional Components, Extensions and remove/add what you wish