If you want to change the first flight: you need to cancel the old ticket, pay any cancellation charges (if any, depends on continent etc), pay travel agent cancel fees (if any).
Then issue a new ticket, if the price went up or down: pay new price. If taxes went up or down: pay new taxes.

Also the first flight needs inside one year after you bought the ticket.

If you use the first coupon(s) then you have 1 year from the first flight, you can change dates (free) and routing (charge).