I booked a round trip ticket, say
A <--> B.
A --> B is used. Now, can I change the return date of
B --> A?
PS: The ticket is bought on an agency and its airline is operated by Alitalia.
I've often changed my return date, due to work; with Alitalia, it was a minor charge. With KLM, it once actually refunded me some of the cost of the ticket, because it was a cheaper return date. However, many airlines have restricted tickets that don't allow changes. You can always call them to verify.
You can ALWAYS change the flight, but the more important question is: "how much do you have to pay for it". This depends heavily on the conditions and fare class of the original ticket. Cheaper tickets typically come with higher change fees.
In some cases it may turn out that just forfeiting the original return and buying a new one way is cheaper then changing the existing one, so you need to run the numbers.