Completed Activities - Allowing Edit or Re-open

All,

Can anyone help

We have a requirement to 'Open' or 'Edit' COMPLETED activities. We have a situation where I need to delete a number of duplicated BPs, before i can do this I need to change the activities from one to the other, I am unable to do this as I can't edit the completed activities to change the ACTIVITY PARTNER

Is there somewhere in config where you can allow edit function within this status???

Kind Regards

Barry