Update only time from my Datetime field in sql

DECLARE @Time as TIME;
DECLARE @Date as DATETIME;

SELECT @Time = CONVERT(TIME, '2016-01-01 09:30:00.000');
SELECT @Date = CONVERT(date, GETDATE()); --2017-03-10 16:37:34.403

SELECT DATEADD(MINUTE, DATEPART(MINUTE, @Time), DATEADD(HH, DATEPART(HOUR, @Time), @Date))

OUTPUT: 2017-03-10 09:30:00.000

UPDATE MyTable 
SET MyDate = DATEADD(HOUR, 4, CAST(CAST(MyDate AS DATE) AS DATETIME)) 

Or this

UPDATE MyTable 
SET MyDate = DATEADD(HOUR, 4, CAST(FLOOR(CAST(MyDate AS FLOAT)) AS DATETIME))

UPDATE TABLE_NAME SET DATETIME_FIELD = CAST(CAST(CONVERT(DATE, DATETIME_FIELD,101) AS VARCHAR) + ' 2' + ':' +  '22' AS DATETIME) WHERE (OUR_CONDTTION)

User the below one to update the Time only for datetime column -

select  CONVERT(varchar(10),datecolumn, 120) + ' 12:34:56', 
        CONVERT(varchar(10),datecolumn, 120) + ' 00:00:00', 
        RSLV_LTR_INITIAL_DUE_DT, *
from twhere able1