Tasks in Engager are great - they're your 'to do' your 'reminders'. If you've not started using these in your Engager account so far, we would recommend reading through our guide - https://help.engager.app/hc/en-gb/articles/21755944428818-Tasks-Reminders-and-Managing-your-To-Do-List
Tasks can repeat, they can be assigned to other team members, they can link to emails - they're magical
Once a task has been completed and hasn't been set to 'repeat', then the task has served its purpose and it's done - you won't be able to revive or edit / undo
You can however, always track 'completed tasks' so simply head to your Jobs page and filter Job Types - Tasks - Due State - Completed
You'll now see your Completed Tasks in Blue
You can client into the Task and Comment and Delete, but you won't be able to revive / create new.
Remember though, a completed task will still sit against the client's record - you just need to make sure that you have the 'completed custom tasks' enabled in your filters.
In the example below, we can see a company and all Jobs, outstanding and completed: