The button is a symbol. I believe that you must edit the text in the symbol?
Double click the symbol, edit the text, and you're ready!
If this is a special symbol, then open Window > Symbol Properties, check the "Label" property of the symbold, edit it, done!
See if this helps you -- if not, please, post the PNG with the symbol here!
You don't say what kind of a button? Is it a symbol or a flattened bitmap? If a bitmap then not many choices but to redo. If the image has a solid color around the text then you can use the marquee tool to select, fill with color then re-type text. Can you send a sample of what you are working with?
Joyce J. Evans