Your hosting account is currently active. What are you attempting to do that constantly takes you to the sign-in page?
As for changing your domain name, you can choose one out three options:
1. You can change your main domain in your account panel. It's located in your hosting details.
2. You can use the addon domain link in your cPanel, if you want to use your domain as a separate website (this is how this option is usually used in website organization. It'll create a subdomain automatically for the domain to point to.)
3. You can use the parked domain link in your cPanel, if you want to user you domain as an additional domain that points to the existing main domain's website (this give you the option of using different domain names for the same website.)