See:
Assuming you know the name of the file you've stored, you can overwrite the file by saving a file with the same name, just with a different Text attribute.
Note that you can only save to the external storage and ASD on a non-rooted phone. Saving to and reading from external storage requires the WRITE_EXTERNAL_STORAGE and READ_EXTERNAL_STORAGE permissions (from the link at the top of this post).
If your phone is rooted, you cannot modify the internal storage with your app.