You can find all of your 'Completed Tasks' in the Settings of your Any.do account. Tap on the Settings tab → Completed Tasks (found under the Tasks section).
Please note that tasks are stored in your Completed Tasks list until you decide to permanently remove them from your Any.do account. You can choose to remove individual tasks or select 'Delete All' to clear your list. These deletions are permanent, so make sure you don't need them for reference before deleting them.