When you are on the home page on the app you can see a list of tasks. On any one of those you can slide to the left and you'll get three options. 1) Update 2) Move to Requests and 3) Done. Selecting Done will by default set to the task to 100% complete.
You can open the task and press the plus button to make a new update. Click the slide icon on the bottom left of the update and you'll have the option to change statuses, conditions, commit dates, and percent complete.
You can open the task and select the details. Then mark the task as done.