Depends....
If you saved the button as a PSP file and preserved all the layers, delete the text layer and re-create it with the new color.
Or, hide all the other layers and do a "color replace" on the text. This can be a bit tricky and may not produce the results you want.
If all you have is a single layer image (jpg or gif) then I'm pretty sure that your only option is the "color relace" function.
There's always a better way. The fun is trying to find it!