How to update column value with auto increment number or unique number

Using this query we can update any column value with any unique number with n number of  digit.
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DECLARE @counter varchar(max)
SET @counter = 1000000000
UPDATE TableName
SET @counter = ColumnName = @counter + 1

Using this query you can update with string character to any of the column value
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Put table name , column name and give a identity column value.

UPDATE TableName
SET ColumnName = 'Test_User_' + convert(varchar,IdentityColumnName)
SELECT * FROM TableName