updating a single field is the same as updating multriple fields.
look at a standard update page,
there is a hidden form element that passes the ID of the record to update.
in the update behavior, the key column is set and bound to the field that passes the ID.
on the bindings panel, the columns to be updated are bound to the corresponding form element, columns that are not bound are not effected by the update, so you can bind only the one column that you wish to update to a form element to update just that one column.