You can change your subdomain name at any time you want. For more details read below.
Log in to your User Panel and go to 'Settings'.
Choose ‘Domain’. Here you can buy, connect, and manage domains associated with your website.
Under the 'Photler domain' you have written your current domain's name by default. Enter there new domain name you want to have.
Click 'Change' button in 'Photler domain' area to save changes.