Click into Settings > Basics.
Enter the new subdomain you want to change to and click on 'Change subdomain'. The change will occur instantaneously and your previous subdomain will redirect to your new one.
Note: We currently don't support the ability to use your own domain (e.g. mydomain.com) for your Skedda account. This is primarily for security reasons (i.e. the requirement for a valid SSL certificate for secure HTTPS communication).