sql how to replace the time value in datetime column

update Table1
SET StartDateTime = CONVERT(datetime,
CONVERT(VARCHAR(10),StartDateTime , 101)
+' '+CONVERT(VARCHAR,datepart(HH,NewStartTime))
+':'+CONVERT(VARCHAR,datepart(MI,NewStartTime))
+':'+CONVERT(VARCHAR,datepart(SS,NewStartTime))
+':'+CONVERT(VARCHAR,datepart(Ms,NewStartTime))
)