If you are looking into either add a custom domain (site address) to your site or simply change the free WordPress.com subdomain you have you would need to log into your account from a browser to do so.
So the first step is logging into your account here: https://WordPress.com/login
Change a WordPress.com site address or domain
You can change your WordPress.com site’s free address (ending with .wordpress.com) in a web browser with this guide:
Change a Site Address
You can also add a custom domain to your WordPress.com site on your Domains page:
Custom domains replace your free address (ending with .wordpress.com) with a domain of your choice, like yourgroovydomain.com.
Learn more about custom domains.
Change a self-hosted site URL
Before changing your self-hosted site’s URL, read this guide carefully:
Changing The Site URL
If you need help before or after changing your self-hosted site’s URL, please contact your website hosting provider for assistance. The WordPress app support team cannot resolve problems changing a self-hosted site’s URL.