To edit a track, open the Album Composer, then look at the tabbed list on the left, and click the track you want to edit. You will notice the main pane on the right shows the track details in an editable form. To edit a field, click on it and it will become editable.
To switch between different tracks just click them on the left tabbed menu.
Be sure to review the meta-data style guide to learn more about the fields on this form.
To replace files, on the Album Editor, choose a track and then a "replace" icon will appear if the track already has an audio file assigned. Click "replace" and choose the new file from your hard disk. Clips with ID3's and embedded artwork will be regenerated.