In the bar above the list click on "Sort list" or on Smartphones on the icon with the numbered list on the far left. This activates the sorting mode. You can then simply move the individual things/tasks within the list. Hold down the left mouse button on the thing you want to move and drag it to the new position. On smartphones touch the thing for a short time and drag it to the new list position. A green placeholder always shows you where the thing will be placed. The change will be saved immediately. Finish sorting to return to normal mode.