admin answers:

You can only change the registrant’s name (and other Whois info) through the registrar where you registered the domain name. Because you are the owner and are responsible for keeping your Whois information current and accurate, you can change any of the Whois info at any time you wish.

If you purchased the domain through your web host and they did not send you information on how to manage your domain (for renewals, Whois info, DNS settings, etc.), you can do a Whois search (I recommend whois.sc) to find out which registrar has your domain, then contact them for your login info. If that doesn’t help, you will need to contact your web host to get that information.