Ok,
For #1

ull down the field from Table B, and type a "1" (In quotes) in its criteria
For #2: There's two things you can do:
1. Pull down the field from table B, and reference to the field in table A, such as
[Tables]![TableA]![YourField]
or
2. Add table A to the query window and create a join between the two fields of interest. Do that by dragging the field from one of the tables on top of the field in the other table, then click ok when the join type box pops up. This will ensure that the update query will only run on records that match at that field.
Make sure that the criteria for both fields are on the same line in the criteria section of the query, or you'll wind up with an "or" statement in the SQL instead of an "AND" statement.
I probably should have warned you earlier, but its always a good idea to backup your table before running an update query. If you do something wrong, you can seriously screw up your data. (Hope it's not too late to warn you!)
Let me know how that goes..
-Patrick
Nine times out of ten, the simplest solution is the best one.