I am facing the issue of grand total becoming zero after placing the order in magento2. Some 3rd party module changes that. I want to know how to recollect the grand total again and set it as while placing the order. So it can avoid the conflict.
First of all, you need to disable that extension which causing an issue. Second thing for updating total for a placed order, you need to edit orders from the backend and save that order. So it will update cancel the current order and create a new one with same information.
I hope this will help you.
Please let me know if you have any query.