I think that this is a change that can be perfomed only by odoo staff (for the odoo online) and that it canNOT be done through your subscription settings ( seems like every subdomain that is related to a subscription and has the form 'subdomain.odoo.com' is internally related to a database with the same name )
So if it is a trial subscription I think that once you choose a name you cannot change it unless you get a paid subscription
From 'Odoo Pricing' page:
Can I use my own domain name?
Yes, we can set up your own domain name for Odoo Standard or Business (paid subscriptions only), e.g: www.mycompany.com. We can also manage email addresses.