I can’t edit DONE reminder & make it ACTIVE again.

If you tap “Edit” on DONE page, This only you can edit note of “DONE history” and it won’t ring. (You can’t select future date).

If you want to re-use a DONE item as an ACTIVE reminder, tap “Clone”.

Now you can create new ACTIVE reminder (which will Alert on the time you selected) with the same title, same note. Just re-select future date & time that you want to get alerted.

Also you re-select Optional settings if you need it.

From Ver.15.3 You can replace Clone with Recycle.
Recycle will remove Done history when cloning a new reminder.