+=item update_payby HASHREF
+
+Updates stored payment information. Takes a hash reference with the same
+keys as insert_payby, as well as B<custpaybynum> to specify which record
+to update. All keys except B<session_id> and B<custpaybynum> are optional;
+if omitted, the previous values in the record will be preserved.
+
+If there is an error, returns a hash reference with a single key, B<error>,
+otherwise returns a hash reference with a single key, B<custpaybynum>.
+