To delete the incorrect activity entry please go to your Walker Tracker page. Scroll down to the date in question. Next, click on the small pencil icon next to the entry. Click 'convert an activity'. Click the small light grey x next to the entry you want to delete. Click the green 'update entry' button.
To enter the correct activity click the green add entry button. Click on the calendar icon and choose the date for which you'd like the entry to be for. Click "convert activity", choose activity from the drop down menu and enter the time for the activity, click "calculate", click "record activity" and then click "Update entry". Ta da!