Update Query question

I have a table that contains inventory on hand balances for different inventory locations. Right now when I have to move inventory to a different location, I have to move one item at a time through our system. I'd like to be able to move the contents of one location to the other location.

Here's what my table looks like:

Item     Location      On hand

1           A1               5

1           B4               1

I'd like to write a query that would move the QTY of 5 from location A1 (resulting in 0 on hand in A1) and add it to location B4 (totaling a QTY of 6). 

Any ideas?

Thanks!