PNG files are raster images. Merely a collection of pixels. What the PNG displays, via its pixels, can be anything from a person's portrait to a block of text. Nonetheless, it's all still merely pixels.
In order to edit pixels you need to paint or draw, you can not use any type tools. Type, in a PNG is not "live" or directly editable. Think of it like a poster hanging on a wall... it may have text on it.... but in order to change that text you would need to paint over the exiting text then create new text on top of the paint over. That is what has to be done to edit type in a PNG file.
There is no simply, easy, quick, method to alter type in a png.
All that being posted, what you have is merely text on a transparent background. All you really need do is create new type and try and work out the layer styles used originally.
(Font's not the same, I know.)
Just playing around for a bit, this is close but not exact.....
If you knew what the font was, that would be helpful in recreating it (just need to add those coloring effects).
If you could get the original source file that would make things much easier.