sql - Update table based on a select from another -

i have select statement below use update quantity of products in table, tablex. cannot seem figure out how match product number query productnumber tablex , add quantity found in statement existing quantity in tablex.

select      p.productnumber, sod.quantity ,so.statecode      salesorderdetail sod right join      productassociation pa on sod.productid = pa.productid left join      product p on pa.associatedproduct = p.productid left join      salesorder on so.salesorderid = sod.salesorderid      so.statecode = '3' 

you can have update base on multiple tables, syntax like

update tablex set tablex.quantity = sod.quantity tablex join product p on tablex.productnumber = p.productnumber join... -- add rest of joins , conditions.