To edit an incorrect entry you can use minus numbers in the distance box. See image below. Please note this will only work if a challenge is not completed.
Please always check the confirmation pop up box, if incorrect do not confirm. If you do and it completes the challenge you will not be able to then edit. What you will the need to do is send us the missing evidence once you have done it on this thread.
Entering a Minus Number
Pop Up Confirmation for a 10Km Challenge
You will see an example of a 10Km challenge with 3KM submitted and no time. 10 - 3 =7KM