We don't use start transaction in SQL Server; it's BEGIN TRANSACTION and COMMIT TRANSACTION (or even ROLLBACK TRANSACTION).
What is the data type of the column "value"? I'm going to assume it's not an integer, which is why you're converting it in the first place. If so, the resultant data type is an integer, so you'll need to convert it back to the original data type to get this to work. e.g.
value = Convert(char(10), Convert(int, value) + 1)
I have done it but got same error as per following
Script line: 4 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'Convert(int, value) + 1)
WHERE code = 'NEXT_CONTENTID';