Insert Into Column From Another Table Column

I came from DW admin background and new to SQL programming. I have 3 tables:

CUSTOMER - cust_id, cust_name, cust_state
SALES_HEADER - sales_id, cust_id, sales_amount
SALES_DETAIL - prod_id, sales_id, prod_name, prod_price

I am trying to insert into SALES_HEADER table from CUSTOMER and SALES_DETAIL tables. Here are the SQL queries that I am trying to insert.

insert into dbo.SALES_HEADER (SALES_ID,SALES_AMOUNT)
select SALES_ID, sum(PROD_PRICE)
from BOBJ.dbo.SALES_DETAIL
group by SALES_ID;

The above query is working good & to get cust_id from CUSTOMER:

Update dbo.SALES_HEADER
set CUST_ID = (select CUST_ID from dbo.CUSTOMER
where dbo.SALES_HEADER.CUST_ID = dbo.CUSTOMER.CUST_ID)
where CUST_ID is null;

This query is not working & getting message - 6 rows updated successfully.

But using select * from dbo.SALES_HEADER, CUST_ID is still showing as NULL.

What did I do wrong and is there a way to write single query rather than two queries.