If you don't have the Business Policies enabled on your account, where you can create and save different policies etc, the only way I know how to create a new policy for future listings is to create a new listing and adjust the policy in that one, then future listings will have that set as the default policy. If you use Turbo Lister, you could also create a new template (or edit existing ones) with the new policy.
You can also change the existing returns policy on current listings in bulk via Selling Manager, but AFAIK there's no global policy setting unless the Business Policies option has been implemented on your account.
