Your brand's Fallback URL is set when you first create your account. You'll see it here in Settings › Branding:
In order to change it, you'll need to contact us directly and let us know what you'd like to change it to. Depending on the availability of that secure domain, we can make the change pretty quickly.
If you'd like to have your brand's Fallback URL changed, reach out to the Workspace Support team from the Support menu at the top of your account!