You will need two things, the login information for your Domain Name Control Panel, and the link for the Control Panel site. If you do not have the login information, it can be emailed to you, but only to the address listed for the Admin contact for the Domain. If you do not have access to that mailbox, you are going to have a problem. Changing the Admin contact email is difficult and time consuming, so I hope you don’t have to do this.
If you need the login info sent to you, the link to the Control Panel, or the Admin contact email address changed, you will have to contact the Support department for your Domain, listed in the Whois information for the Domain which you can find by doing a lookup at http://whois.domaintools.com. It will be listed as the Registrar. You should find the contact details on their website.
Once you have the login and link, log in to the Domain Management Control Panel and make the changes in there. Some TLDs require additional confirmation of certain changes, for instance if it is a .ca Domain Name you may have to also get the CIRA login information and confirm your changes with them, especially if you are changing the Admin contact info or the Nameservers.
If you need more help, you can get in touch with me at http://www.webelf.ca