Update table using other table's info

[New2This]

Hi
I have a table CustOrder in where a field called RequiredDate needs to be updated. This table holds all orders for customers (CustID). Within this table there is a field called ReleaseNo. The tables are as follows

ReleaseDates:
ReleaseNo
PostalCode
ReleaseDate


Customer table:
CustId
Name
Address1
PostalCode

CustOrder:
CustID
OrderNo
ReleaseNo
RequiredDate

For each order a RequiredDate must be entered based on the ReleaseNo and the customers PostalCode. Every field in each of these tables are filled except the RequiredDate field.
I thought I would write an update query to fill it with the proper dates based on the above criteria

How how how do I right the update query
Seriously stuck

thanks
Mary