Hello AramA -
You can make Bulk Changes to your Saved Listings using the Advanced Edit button on the Saved Listings page of your account. For this, it sounds like what you need to change is your returns, so after selecting the saved listings you wish to update, click on Advanced Edit.
On the page that ensues, from the Field pull down menu, select "Return Policy" and the appropriate eBay site for your listings. Then in the pulldown for Return Policy select "Returns Accepted" and other options will appear. You can make your adjustment using the field for "Item must be returned within:" and then click on the Continue button.
Alternately you can create a Checkout profile from under the Profiles menu, set this up per your preferences and then use the Checkout Fields option from the Field pulldown menu on the Advanced Edit page to apply that profile to your selected Saved Listings.
If you run into any further difficulty, please feel free to file a support case from under the Help tab so we can assist you further.