In the Home screen (shown below), you can see all your Open Tasks from Salesforce.
Tap on the highlighted section to see all Open tasks (shown below).
To mark a Task as Complete, swipe left (shown above). Tap on the + sign (top right corner) to create a new Task.
Tap on a Task to see details (shown below). If you want to edit the Task, tap on Edit (top right corner). Swipe left to mark the Task as Complate.