Update existing rows with squence number/char or any unique data I added a new column in the Table X

This column "cn" has to be unique and mandatory, but old data don't have any value.

How to update the existing records with sequecely or random unique data?

Thank you.
If you are happy with a number starting from 1 you can use row_number().

update T
set cn = rn
from (
       select cn,
              row_number() over(order by (select 1)) as rn
       from TableX
     ) T