1. Log into your Merchant Center and navigate to the "Contacts" tab
2. Click "Show Filters" next to the search bar
3. Click the gray drop-down menu that appears and choose "Lists"
4. Click the purple text at the end of the sentence that appears and choose one of the lists you would like to combine from the drop-down menu
5. Click the purple "Filter button"
6. Scroll to the bottom of the page and click "Download .csv" in the bottom left-hand corner
7. Once the list has been downloaded, follow the steps to upload your contact list file (make sure to choose the second list's name from the drop-down menu on the Final Confirmation page): How To: Upload Contact Lists
8. Once you have uploaded the first list file into the second list, follow the steps to delete the first list: How To: Delete a Segmented Contact List