We do not currently have a way to change the domain on an existing WordPress site at this time. These are the steps you need to take in order to "change" the domain on an existing WordPress site.
Add the new domain via https://wpo.bigscoots.com/sites/add-site ensuring to choose the Clone existing WordPress Install and select the domain you want to change.
Once the new domain has completed being cloned and is live / visible on the internet, you can then proceed to remove the old domain from the account.
Note: If you want to redirect the old domain to the new domain, you can do this by adding the old domain into the Domains tab when viewing the new domain as shown here: