how do i update mulitple rows with dynamic data?
UPDATE item 
SET Qty = Qty -
(SELECT sum(Bask_Qty) FROM user_basket 
 Where item.item_ID = user_basket.item_ID and user_basket.user_ID = 1);

This would update all items, though you may not notice it ( minus zero! ).
Better add your
WHERE item_ID IN (SELECT item_ID FROM user_basket WHERE user_ID = 1);