--Table
CREATE TABLE LineNums (INVOICE int, linenum int)
--Data
INSERT INTO LineNums (INVOICE) VALUES (12345)
INSERT INTO LineNums (INVOICE) VALUES (12345)
INSERT INTO LineNums (INVOICE) VALUES (12346)
INSERT INTO LineNums (INVOICE) VALUES (12347)
INSERT INTO LineNums (INVOICE) VALUES (12347)
INSERT INTO LineNums (INVOICE) VALUES (12347);
--Update
WITH CTE AS
(SELECT INVOICE, linenum, ROW_NUMBER() OVER (PARTITION BY INVOICE ORDER BY INVOICE) AS linenum2
FROM LineNums)
UPDATE CTE SET linenum = linenum2
--Test it
SELECT * FROM LineNums
John