updating records in table A from selected records in table B

I've pored over the syntax for UPDATE but I think I'm missing something.

Assuming a schema such as:

      Column       |            Type             |
    Modifiers
--------------------+-----------------------------+------------------------------------------------------------
 id                 | integer                     | not null default
nextval('service_bills_id_seq'::regclass)
 fk                 | integer                     |
 start_time         | timestamp without time zone |
 quantity           | numeric(10,5)               |
 cost               | numeric(10,5)               |

Starting with the results from this query:

     SELECT candidates.quantity, candidates.cost
         FROM table_b AS candidates
INNER JOIN table_a AS incumbents
             ON incumbents.fk = candidates.fk
           AND incumbents.start_time = candidates.start_time

... is there a way to update quantity and cost fields in incumbents
with the matching records from candidates?  It seems that UPDATE is
designed only to update one record at a time...