Sounds like you want to UPDATE and not APPEND. Append adds records while
update changes values in a field.
BACKUP the database. BACKUP the database.
Create an unmatched query using a common field like SomethingID. Open the
query in design view, click on the line connecting the tables, then double
click on the line. Select the top option - where fields are equal. Remove
the 'Is Null' from the Criteria row.
Add the field to be updated and to be used to perform the updating.
Add criteria. Run query and review data.
Copy the field to be used to perform the updating and paste inside of
brackets in the Update To row under the field to be updated. If the field
names are the same then include the table name also like this --
[TableName].[FieldName]