Of course! Simply pick the task you want to dump and tap the trash icon.
Keep in mind that you can dump tasks during seasonal regattas in two different situations: 1. You've already picked a task, but now you want to dump it. If you dump it, the task is still counted as used and is subtracted from your task quota. You can find the list of completed and dumped tasks in the Task Journal located in the Tasks tab of the regatta window. 2. You haven't picked the task, and it's displayed in the task list in the Tasks tab. You can only dump tasks from the task list if you're a Leader, Co-leader, or Elder. In this case, the number of the tasks available for each participant remains unchanged.
Interseasonal regattas have the following rules for dumping tasks: 1. You can dump any task you've accepted regardless of your rank, but you'll lose the task progress. There is a waiting period if you want to choose the task again—however, this does not apply to other co-op members who can choose the task immediately. 2. Co-op leaders, co-leaders, and elders can remove the chests no one is working on. If at least one of the chest tasks has been picked, only leaders and co-leaders can remove the chest.