Here you can view the existing lists, with their list code and list GUID.
The button "Actions" far to the left allows you to edit a list or to delete it
: 
Let's see all the options you have when editing a list:
When clicking on "Edit", you will be redirected to the list properties section.
To start with, you can give the list a name and indicate a company (e.g. if you are an advertisement agency and you are managing the mailings of several clients).
Then you have 6 tabs; visit this page for a detailed description of each tab.
Deleting an existing List
It is important to note that deleting a List is different from removing a List. Deleting completely erases the List data from the MailUp database, including the unsubscribes, filters, groups, triggered messages, statistics, etc.Therefore, be sure to export any unsubscribe data before deleting it from the List to make sure any of those addresses, if re-imported into MailUp, are re-imported as unsubscribed.