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...