I want to update a column only the date without the time change

If you mean that you have a column containing DateTime information, and you want to change the date portion and leave the time unchanged, that's quite cumbersome, because there is no easy way to do it.

UPDATE MyTable SET myDateColumn=CONVERT(DATETIME,'2014-02-12') + CAST(myDateColumn AS TIME)

CREATE TABLE dbo.MyTable(
    ID INT IDENTITY(1,1) PRIMARY KEY,
    DTColumn DATETIME NOT NULL
);
GO
INSERT dbo.MyTable(DTColumn) VALUES 
('20120410 08:03:00.000'),
('20010101 01:01:01.000');

BEGIN TRANSACTION;
    UPDATE  dbo.MyTable
    -- Style 126 / ISO8601 = yyyy-mm-ddThh:mi:ss.mmm        
    SET     DTColumn = STUFF(CONVERT(VARCHAR(50),DTColumn,126) ,1, 10, '2012-05-10') 
    -- Uncomment this line to see the old and new values
    -- OUTPUT   deleted.DTColumn AS OldValue, inserted.DTColumn AS NewValue
    WHERE   CONVERT(DATE,DTColumn)='2012-04-10' 

    SELECT * FROM dbo.MyTable;
ROLLBACK;
-- COMMIT

Let say the new date is current date using getdate()
we can do this to get the date part:

convert(varchar, getdate(), 101)

we can also do this to get the time part of the old date time:

convert(varchar, datetimefield, 114)

So the idea is to concatenate the date part of a new date time with the time part of the old date time:

update table1 set datetimefield =
(convert(varchar, getdate(), 101) + ' ' + convert(varchar, datetimefield, 114))

As for the meanings of 101 and 104, refer this for more detail:
sql-convert[^]

For example pls find below
update employees
set date_of_join = dateadd(dd,1,convert(varchar(10),date_of_join,102))+convert(varchar(10),date_of_join,108)
where employeeid = 101