You can only update 3 fields on a campaign once it's been created:
UserGuid- A pass-through field that should tie to a unique user ID in your system
ExpiresAt- The date we will stop recording data on a campaign, note this time is UTC and must be between 1970-01-01T00:00:01 and 2038-01-19T03:14:07.
NeverExpires- If this is set to true, Litmus will never stop recording data and will ignore
The response is the campaign in json format with the updated values.