Most uploaded shields have had their text converted to paths to permit them to display correctly. Unfortunately, this makes them more difficult to edit. You can instead download and modify these template shields. (You will need to convert the new text to path before uploading; in Inkscape, the correct shortcut is Ctrl + Shift + C.)
To use these shields, you need a vector editing program, such as Adobe Illustrator or Inkscape (which is free). In most cases the template will have the right font set up for you.
These shields should never be used in article space.
Wikimedia Commons has media related to Highway shield templates.