Question: I have winXP LibreOffice and MS office installed.

How do I make LibreOffice the default opening program for all of its know document types?
I know that I can change one extension at a time. But is there a way to change all of them?

I want LibreOffice to open .doc .xls and ... (I do not know all of them)

Answer: 
To make office libre the default reader for all your documents:
Right click any document, tap Properties. A box opens that enables you to change the opener to office libre. Do this for a couple documents and OL magically becomes the default opener for all docs of the same type This works for windows 10 at least.
For me it worked to just use the "default apps" in windows system settings.

just hit start and type default apps