To change a single document, click the "Text Options" icon in the
toolbar and enable the "Auto-Expand Tabs" option. This expands all tab
key presses to a number of spaces. To change the number of spaces,
open the font window with View -> "Text Display" -> "Show Fonts" and
set the tab width at the bottom. To convert tabs already existing in
your document, use the Text -> Detab... command.
To use spaces as the default for all new documents, go to Preferences -
> Editor Defaults and set your default font and number of spaces per
tab. Then enable the "Auto-expand tabs" checkbox that appears on the
same preference pane.
Hope this helps.
-Dennis