How to update the value (number) by the respective quintile number in a table by using the NTILE function ? (Transact-SQL)

Transact-SQL
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5

Yup, that's fine.

Tell me, what are you trying to accomplish here:

set @CDate =ntile

That's invalid syntax of course. NTILE returns bigint, not date. Also, maybe think up a different alias. It's confusing when you use a built-in function name like ntile for a column alias.