Set day (1-31) portion only for date column in oracle -

i need set day portion of date in oracle retain month , year. can tell me do?

e.g. if date 01-jan-15 want set day 15 15-jan-15.

    1* select sysdate, trunc(sysdate, 'month')+14 dual   sql> /    sysdate     trunc(sysda   ----------- -----------   25-sep-2015 15-sep-2015    sql>