Generally, you can't edit the campaign members of a Contacts block in a Flow. This is manily because the contacts are already part of the Flow journey so it's best to leave them as they are. But if your Flow is scheduled (daily/weekly/monthly), then by using 'Freeze' you can change the Campaign Members to a different segment. You can do this by entering the Flow and:
1. From the Activate tab, click on the Campaign status and change the status to FREEZE
2. Hover over the Contacts block and click on the settings button.
3. In the Campaign members dropdown, select the desired segment group.
Note: a text pops up under the selected campaign members that says “Resuming from Freeze to Active flow state won’t use the new or updated segment unless the flow is scheduled or repeated in the future”.
If you've done this, the next scheduled activation will use the new contacts instead. Perfection!