What you are trying to do is not an easy job, in my opinion.
Firstly you need a matching font, then remove the existing text, possibly with the clone tool or even paint by sampling nearby colours like in this tutorial:
www.youtube.com/watch?v=kSxuSgch7-YThe billboard is a tough one because it has a lot of shades in it.
You could also try to make the new text out of the existing image, by copying und cutting individual letters out.
But that requires that you can find all the letters you'll need for the replacement text.
Its also possible to replace the whole billboard by something made in Gimp.
Then you could use any font, with a grid and possibly the perspective tool.
The shaded texture is still a tough one.