Once you log into the Roamli portal you can select the Event you wish to edit from your active list. Once you click on the event it will open and you'll see the current statistics along the header along with the Challenges listed underneath.

In the top right corner, click on "Details" which will allow you to update the event information.

At that point you'll see the descriptions to update just like when you first created the event. Feel free to modify any section including Challenges and then scroll down to select "Apply Changes" once complete. They will auto-save and your Event will be refreshed once you return to the dashboard.