SP With Inner Join - Update Table Value (Single Column)

I'm trying to do the following:

update a tables value(a single column), based on a query of another database, using a common column.

Here's what I've cooked up so far:

Declare @trackingNo nvarchar (50)
Set @trackingNo =
(
select tracking_no from P21_Shipping.dbo.shipping_data t1
inner join P21.dbo.oe_hdr t2 on t1.order_no = t2.order_no

[Code] ...

print @trackingNoThe error it's returning is:

Msg 512, Level 16, State 1, Line 3
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

So, I'm wanting to query Shipping_data for order_No that matches the same orderNo column value in P21 Database(oe_hdr table), then update P21 oe_hdr table with "trackingNo from Shipping_data