Transfer Data from One Table to Another or else Update the Data
You can use INSERT ON DUPLICATE KEY UPDATE. For this it is necessary to have a unique index on GeneralInventory.ItemCode, so that duplicates can be detected!

Your statement would look like the following:

INSERT INTO GeneralInventory (ItemCode, Qty)
SELECT RE.ItemCode, RE.ReceivedQty
FROM Receiving RE
WHERE RE.RINo = 'Data of RINo'
ON DUPLICATE KEY UPDATE
SET Qty = Qty + VALUES(Qty);


A quote from the above linked manual page to explain the VALUES() function:


  You can use the VALUES(col_name) function in the UPDATE clause to refer to column values from the INSERT portion of the INSERT ... ON DUPLICATE KEY UPDATE statement. In other words, VALUES(col_name) in the ON DUPLICATE KEY UPDATE clause refers to the value of col_name that would be inserted, had no duplicate-key conflict occurred.