How to change only time while keeping the date same in timestamp type
Add the desired INTERVAL

select date + interval '5 hour'


Have a look at: Date/Time Functions and Operators on Postgres documentation.

select a.MyDate + interval '5 hour' as Result_Date
from (select '2017-07-01 00:00:00'::date myDate) a

|result_date        |
|:------------------|
|2017-07-01 05:00:00|


dbfiddle here