how to associate file types with applications

[John R. Sowden]

When I try to open a Libreoffice text file, I am given the leafpad 
option.  I cannot find where to change the applications associated with 
files types.  There is a 'settings manager' application to set the 
default browser, email, file manager and terminal emulator, but no others.

I have asked before, a local (meaning on my computer) help system would 
be 'help' ful.

Help?

John