How can we change a return date on a round trip ticket on AVE train?

I have never needed to do such a thing but if you are here - http://www.renfe.com/EN/viajeros/index.html - it would seem you click on CHANGE.

Refunds (and partial refunds) are only available for a certain type of tickets, so first thing is to check which ticket you got and see if you're entitled to any refund at all and/or if you can indeed make any changes. The best way to check that would certainly be over the phone, on their dedicated lines: http://www.renfe.com/EN/empresa/atencion_cliente/index.html

But you also can check on the web: try buying another ticket for the same journey, pick the same train (same schedule) and where it says "options" (with a + sign), click on that and it will show you the available fares. Underneath, the section "Conditions" will tell you whether you're entitled or not to any return for cancellation and whether you can make changes. Check that against the ticket you already purchased. Even in the English version of that crappy web the wording in this section is in Spanish, but suffices to know that "Cambios" means 'changes' and "Anulaciones" means 'cancelations'. The % means the 'cost'.

... or as said earlier... simply try to actually make a change thru the option in the menu mentioned above.

The first thing to know is the ticket changeable? Many tickets purchased on-ine early are discounted tickets with restrictions so you need to understand what you have purchased.