I find the flow of this a bit confusing, so I took some brief notes as I did it today for a buddy pass thinking it might be of help to someone.
I’m going to assume you are logged in and are looking at the TravelNet landing page
1. Click Manage My Account
2. Click Find Listings or Reservations
3. Under Standby Listings, Click the Record Locator of the Flight you want to change
4. This will bring up your current flights. Click Change Listing
5. Click “Add More Flights” button
6. I’m going to assume you only want to change one leg of the flight, so change From/To city (airport code) and to one way and the date on the calendar, then click Search button
7. You may get a warm weather warning nag message, click continue listing button
8. This will bring up all the flights for that day. Choose the flight you want to change too, then click, Continue to Listing button.
9. Next page is the weirdness (at least to me) It will now show the flight your changing, the flight your adding, and maybe your return flight. In the far right, next to the flight info, you will see in blue, cancel next to the flight you are changing, remove next the the one you are adding, and cancel next to the return flight (if it exists). You will click the blue “cancel” next to the flight you no longer want. After that, the blue cancel becomes the word confirm. That’s all you do. Don’t click any other blue words. Add your email address in additional information to get confirmation of changes, then click continue
Hope this helps.