The goal of an UPDATE query is to modify the contents of existing records in a table.
The UPDATE query must specify the following:
the subset of records in the table that will be updated
the field or fields whose contents will be modified
the new contents of those fields
There are three basic steps to working with columns in an UPDATE query:
Add fields to your query.
Your query must include at least one field whose contents may be changed by the query.
You can also include one or more fields that will be used to determine which records will be updated by the query, even if the contents of these fields themselves will not be changed.
Specify the new contents of the fields to be modified.
If the changes should only be applied to a subset of records in the table, set criteria to filter out the records that will be modified.
More:
Adding columns to the UPDATE query