How to edit the completion time on completed task?

"The task must be reviewed before completion" (How the "In review"-status on tasks works)

You can edit the date of completion of the task or other information of the task or, for example, the information of the task form, only if/when it has been defined for the task when it was created that the task will go through "In review"-status after completion (and not directly to the "Completed"-status).
task must be reviewed

You can set the completion date in the "Completion comments" tab, which can be found behind the three dots (if not visible).

From a quality point of view, it is not appropriate that, for example, the completion date of the task can be edited afterwards.

If, for one reason or another, the task has not been completed on the day when the work is actually completed, the correct time of completion of the task can be communicated in the comments of the completion of the task or via a message to the device in question.