To change the association of Microsoft Office file name extensions to open the files either in LibreOffice or in Microsoft Office, using Microsoft Windows:
In Windows Explorer, right-click a file of the type that you want to assign to another application.
In the context menu, choose Open with - Choose another app.
In the list of applications, select the program that should open the current type of files. Make sure that "Always use this program" is selected.
If these steps do not apply to your brand of Microsoft Windows, search your Microsoft Windows Help for instructions how to change the file associations.