In Word for Windows, you can easily change the default font in the Font dialog box. Press Ctrl+D, choose the settings you want and then click the Set As Default button. When prompted, choose to save the settings in the template (which is the Normal template unless you specifically created the document based on some other template). In the screen shot below, note that I have selected the "This document only" option.
For the Line Spacing, I recommend that you make the modifications through the Modify Style dialog box for the style you are using. If you haven't explicitly selected some other style, the pre-selected style is usually Normal.
Press Ctrl+Shift+S to display the Apply Styles pane. Click Modify. In the dialog box, access Format > Paragraph, from where you get access to the Spacing options as well as the alignment options in the Paragraph dialog box. See screen shot.
In the Modify Style dialog box, you can copy the modified style definition to the attached template by selecting the radio button "New documents based on this template."