Cast your ntext to nvarchar(max) and do the upper and left operations. Sample below.
SELECT UPPER(LEFT(cast(Comments as nvarchar(max)),1)) +
LOWER(SUBSTRING(cast(Comments as nvarchar(max)),2,
LEN(cast(Comments as nvarchar(max)))))
FROM dbo.Template_Survey;
Following should work for update.
Update dbo.Template_Survey SET Comments =
UPPER(LEFT(cast(Comments as nvarchar(max)),1)) +
LOWER(SUBSTRING(cast(Comments as nvarchar(max)),2,
LEN(cast(Comments as nvarchar(max)))));