To change the number of dashes used in your Scene Headings.

1. Click Format > Edit Script Formats.
2. On the right, click the Scene Headings button.
3. In the middle of the screen, go to Time-of-Day Dashes and change the number to one.
4. Click OK.

Note: this will only change new scene headings not existing ones.

To change existing Scene Headings use Search > Search & Replace (Search > Find/Replace on Mac).  Search for the Time-of-Day, including the dashes (e.g., "-- NIGHT").  Replace with one dash (e.g., "- NIGHT").