Bring up task's menu by long-pressing it, or tapping it and then tapping "MORE" on the yellow toolbar at the bottom
Choose "Mark done"
Only the top-most task can be mark done. Which means Goalist is a planner and not a traditional to-do list or calendar. Its idea is to use the actual tasks' sequence. The plan shows activities done/planned one-by-one, just like we do it in life. It enables precise scheduling and time tracking.
By top-most I mean: the first active (not done) task - the first one below the date.