To complete a task, you have to find a “Complete” button anywhere near a task. Likely, you will do this from the App Home.
Click button “Complete” under the task you want to complete

You will find the task has disappeared from the list, and the “Completed (0)” counter has incremented to (1).

Task watchers have received a notification about the task completion.

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Go to App Home and click the “Completed (…)” button

Everybody in your workspace who can see a task, can also edit it - that includes being able to complete it.
Task completion can be reversed from the task details modal.
Click task’s overflow menu “⋯” and then “📃 View”

Click “↪️ Undo completed”

As with completing a task, watchers have received a notification message about it.

See: ‣