Yes.
Add both tables to the QBE grid.
Join them on ZIP.
Change the query type to update.
In the first available field cell, select the PCP field of the A table.
In the criteria cell put -
[table B name].PCP
Zip does not seem to be the correct thing to join on since unless this is a table of zip codes, zip will not be unique and you could have multiple values in the PCP field of table B so I don't know what would happen but I'll bet it won't be pretty.