How do I change the style of the default horizontal-line?

I want to be able to alter the thickness of the line, and to make it a single rather than a double line. The only help document I can find on this aspect is out of date.

I’ll suppose you inserted the horizontal line with Insert → Horizontal line. If that’s not the case, edit your original question to add more details about your problem.

Open the style organizer (F11) and look for the Horizontal Line paragraph style: with a right click on it, select the Borders tab and change the settings as you want. If you want all your new documents to follow that setting, save the resulting file as your default template (File → Template → Save as template).