To further add to the previous reply, the best thing you can do is check the server or PC where the web site created and search for .PNG files and open them in Fireworks (downloading them first if they are on server) to see if you can find that file. Then click on the text tool to edit that text.
If you cannot find the .PNG file, you will have to try to rereate a button like it buy trying to find the same font, background color, etc. You will have to check your site in Dreamweaver (or Fireworks if done entirely in FW) to see if the text button has the color below it or if that's a seperate button. very likely whoever drafted the site made two buttons: one called Premises with the blue bar underneath, and one called Premised with the lighter color underneath. Likely they save them as .PNG files first (which you should always do--this is referred to sometimes as your source file because it is editable) THEN exported them as GIFs. Then they likely did a rollover in DW.
Please feel free to reply if I can be of any more assistance.