Especially in competition it often happens rather often that a pilot decides to return to the start line and make a restart.
Everything is calculated automatically in Oudie so restarting the task is as simple as navigating back to the start line. You should select Menu > Task > select the start point > Goto. Everything will be the same as before you crossed the start line for the first time.
Note: In order to restart you must select the start point from the Menu > Task page. The goto point behaves differently when you navigate towards the start line compared to how it behaves if you select it as the goto waypoint (in order to know that you have a safe glide back height). When navigating to the start line you always navigate to the nearest point on the start line which can differ by several kilometers from the actual point.