Clicking the 'MERGE LIST' button shows you the entire list. Select the lists you want to merge by putting a check mark in the box and hit 'Merge' to continue.
Enter a descriptive name of your new merge list, in this example it is named Merge Lists, then hit 'START'
'List Merging has started' pops up, merging process goes in a background, and if lists are big it will take some time to merge. During this time new list will have the "updating" status.