How do I change the date of my train reservation?

For the cheap RES-say up to 10€-normally not. In some countries/cases you MAY be able to excahnge them for a same-same IN that country at a counter.

For the more expensive ones, f.e. also €*-you have to contact where you booked it-and these will also set the rules.

It should be evident to always check first any conditions etc that are usually sent out together with the mail that allocates the seats etc. and not let other tell you to do that first!

That depends on the railway company that issued the reservation. You can read more here.

https://www.interrail.eu/en/support/reservations/can-i-get-a-refund-for-a-reservation-ive-purchased-but-wont-use

Will I get a refund on that reservation 

Normally you can't change the date. There are some exceptions. You normally need to cancel the reservation you have and buy a new reservation.