When in Tagging Mode and in Grid View or Loupe View, the left-hand column of Photosmith will show the metadata fields which are available for edit. These fields include Photo Title, Caption/Description, Location, Creator/Photographer, Copyright, Category, and Job Status fields.
These metadata fields may be edited by tapping on the respective text box, then keying in your information as you would expect.
The one notable exception to the above is Keywords. Tapping on the Keywords field will trigger a special drawer to appear, and is discussed in Section 3.4.2.