Follow these steps to update the time frame for a challenge in the The Conqueror App:
Login into your account and on the main page, find the challenge that you want to update the time frame for and click on "View Challenge".
In your challenge's main page, click on the rocket icon in the top right of the map.
From the dropdown menu, click on the gear icon.
From the options, click on TIMEFRAME and you can edit the start date and time frame, then click Save & Close.
Here are the steps above in action: