URL still displayed as “.wordpress.com” despite having linked to custom domain

Hi all – i've got my own custom domain name that i've linked to my wordpress site – the site itself is linked fine to the domain name, as in when I type the domain name into the browser the site loads, but when I look at the URL, say when i'm on the homepage, it still says ".wordpress.com" instead of "mydomainname.com" – How can I fix this? Thanks