For anyone like me who does not use Turbo Lister:

Go to this page: Selling Manager Pro > Active Listings

Click: Edit > Edit all ### listings (you don't need to check any boxes)

You will be taken to the bulk editing page. Once it is finished loading:

Click the checkbox at the top which will activate the checkbox on all listings.

Click: Edit Fields > Handling Time

Choose: Edit Listings In Bulk

Choose: Change To

Choose: Your new handling time.

Click: Save and Close > Submit Changes > Confirm and Submit > Ok

Done! Hope this helps someone, OP or otherwise.