PNGCommentator is an easy to use application that helps you change the metadata of PNG image files.
The "Add" and "Remove" buttons at the bottom of the window create and delete metadata fields. Information can be added to these fields by typing into the columns of the metadata table.
The dropdown menu beside the "Remove" button contains a series of predefined metadata titles (the ones contained in the PNG specification) and allows you to create those fields quickly.
When you are finished editing, save the image. The revert button will reload the image from disk, taking it back to the state in which it was last saved.