If you are not using Vendio Reviser, once a listing is active any revisions must be done from the listing venue directly. For more information on Vendio Reviser, please reference this URL:
Otherwise, you should find an edit or revise option under seller services on the listing venue. Note that most venues will not allow you to make changes to the item description if the listing is in an auction-type format and there is a bid, but you should be able to add to the description even in this case.
