You can access the export function by logging into your account and going to Contacts >> Show contacts, then either searching for 'All contacts' and 'All time' and selecting all campaigns from the advanced options. If you only want to export selected contacts from your list, you can either specify a time frame in this section, or set a filter for the contacts using the advanced options.
After you have completed these steps, please scroll to the bottom of the page where you will see (under the list that has appeared) the option to export your list to a CSV file.
