Use Menu, Format, Edit Stylesheet and make your change. But I think you want the Face to be Plain, not Regular. This change should alter the look of the entry in the Stylesheet and also in the notebook. If you just want to attach the style to the specific notebook you can close the stylesheet at that point.
If you want to Save the stylesheet for general use in other notebooks, then use Install. This will install the notebook in your
$UserBaseDirectory/Mathematica/SystemFiles/FrontEnd/StyleSheets
folder. It will appear on your Format, Stylesheet menu.
But the stylesheet will still be attached to your notebook. Now use the Stylesheet menu to specify the new installed stylesheet.
If you are sending your notebook to someone then you will also have to send the stylesheet (and tell them where to put it) or you will have to attach it again to your notebook.
If you are also going to have other things, such as a package, and are working on some specific application, then you might want to create an application. These also have a place for stylesheets and can include notebooks so you could just zip up the application and send it to another person. The notebooks would automatically use the stylesheet and you wouldn't have to give any special instructions. I describe how to build an application and stylesheets at my web site. A Mathematical Style. Few people seem interested in this however.