iCloud log in access from my laptop

I have recently lost my iPhone so as a security prcaution I changed my Apple ID and password. This automatically changed the same log in information on the website icloud.com. However, when I start up my laptop and the icloud window opens asking me to sign in, it still shows the old Username, which no longer exists. How to I update this information as I cant get passed the log in window?

Cheers, karelynn