Alter the Computed Column

ALTER COMPUTED COLUMN is not allowed if the compatibility level is 65 or lower. For more checking the compatibility level use sp_dbcmptlevel (Transact-SQL).

so you would need to Drop the computed column and re-create it.

ALTER TABLE [MyTable] DROP COLUMN MyColumn

GO

ALTER TABLE [MyTable] ADD MyColumn AS (A+B+C) PERSISTED