if you are using SQL Server, try (didn't test, so it might not work)
update yourtable t set qty = (select qty from yourtable t2 where item=t.item and month = 1) where month = 2
update yourtable t set qty = (select qty from yourtable t2 where item=t.item and month = 1) where month = 2
set qty=b.qty
from tab1 a,tab1 b
where a.item=b.item and a.month=1 and b.month=2
set a.qty=b.qty
from tab1 a,tab1 b
where a.item=b.item and a.month=1 and b.month=2
看一下内连接的介绍吧