From the Cubs official press release:
All tickets for the postponed game from April 19 will be honored at the gates for the 12:05 p.m. game on July 12. There is no need for fans to exchange tickets.
Fans unable to attend the July 12 make-up game may request a refund by sending their tickets to the Wrigley Field Ticket Office at 1060 W. Addison St., Chicago, IL 60613 or by visiting the Wrigley Field ticket windows during business hours. All refunds must be requested before May 31.
The Cubs will announce an on-sale date for refunded tickets shortly after May 31.
The make-up game scheduled for Sunday, July 12 at 12:05 p.m. is now part of a day-night, separate admission doubleheader. The regularly scheduled game will follow at 7:05 p.m.
Fans are encouraged to visit cubs.com for information regarding this make-up policy.