I have an image which represents a button with text on it. The whole image is contained in one layer.
How could I edit the text that is on the button?
If you use the horizontal Type (T) tool it does not work?
Since what i assume that you are saying is that the text and picture (button) are one layer, you would have to cover up the old text with the button.
You may want to use the "clone stamp tool" or even select the text and use "content aware fill".
Then make a new layer just for the text.
If this won't work for you as there is too much detil that would be lost due to a glossy button etc, you could always try to remake the button.
Check out http://iceflowstudios.com/ for some simple guides to makinging buttons. They also have some free PSD of buttons.