In seller hub, Active listings.

If you don't have postage price selected, click 'customise' on the right and include it.

Select all those items with postage prices you want to chage.

Click 'edit' up the top

Select all items

Click edit fields

Select Postage(domestic)

Change what needs to be changed.

Click 'okay' or variations thereof until your listings update and you are returned to Active listings.

Rinse and repeat until you've done them all.

Note:

If you are editing them all, just select edit and 1-500.