Changing the delivery type in mass for existing contacts can be done using the bulk edit panel.
Selecting the contacts and doing the bulk update
To change the delivery type for multiple contacts, first select the contacts by clicking on Contacts in the top menu and then selecting the ones to be changed using the checkboxes on the left or click the top left checkbox to select all of them.
You can also do a search or filter to look for the contacts that need to be changed.
After doing this, the bulk edit panel will show up. In the panel, under Delivery Type, select Replace existing value with… and then enter the required delivery type.
After selecting the new settings click update.