The dialog launcher is the small arrow in the bottom right corner of the Paragraph group, but you can easily open the same dialog by right-clicking in any paragraph and choosing Paragraph.
Keep in mind that you must not only change the line spacing to Single but also remove the10 points Spacing After. If you want this to apply to new documents, then, when you click Set as Default, you must choose "New documents based on this template."
If all else fails, you can open the Normal template as a document and make the change there:
In Word, press Alt+F11; the Visual Basic Editor opens. Press Ctrl+G to activate the Immediate window. Type
NormalTemplate.OpenAsDocument
and press Enter. Normal opens, but note that it may not be on top of all windows. You can close the Visual Basic Editor. Make your desired changes in the Normal file, and then save and close it. Note that the type of change you are making may not register as
a change that dirties the file (look to see whether the Undo button is activated). If not press Spacebar, then Backspace before saving to make sure the template is actually saved.