Anne, while it's not clear, I think you are referring to your return flight. Here are the rules for changing a restricted (non-refundable) ticket:

On the outbound flight, you cannot change the date of travel, at any cost, without having the ticket re-issued, paying a $75 change fee and then applying the funds from your original ticket toward a whole new ticket. You can standby for any earlier flight on your scheduled day of departure. While the rules state that you can standby for a later flight on your scheduled day of departure, as long as you check in at the airport no more than 2 hours after your scheduled departure, you can see from this thread that CO's computers sometimes cancel the entire reservation if you no-show your flight, requiring untold agony to get everything reinstated.

On the return flight, you can standby for an earlier flight on the same day as ticketed. Alternatively, you can change the return date after paying a $75 change fee and any difference in fare class. In other words, if you are ticketed in one of the least expensive fare classes, depending on the route, the fare difference could be $600-$700, or substantially less, depending on the route. In some cases, the original fare class will be available on your newly chosen day of departure. If that is the case, the only charge will be the $75 change fee.