Why can't I unlock a layer in Express after having been saved as a PNG file?

Created a business card. Saved it in the recommended png format. Realized that one element was misaligned. Uploaded the document, but it now appears as one image. All layers displayed on the side. Naively thought that I could just go and edit the text. It however only shows a new layer on topof the one I think I am unlocking, but it does not let me edit my initial text. Only add another layer on top! It cannot be possible that the layer cannot be edited when coming back in, no???