Your "subdomain" is unique to you, and it forms part of the website address that your landing pages are published to.
You chose your subdomain at registration, but if you want to change please raise a Support Ticket (mentioning your current subdomain and your new subdomain) and we will do our best to change it for you.
Please note that all your existing landing page URLs will change to the new subdomain, so your old links will no longer work.