Login to the control panel - https://store.firstserv.com/clientarea.php » Account Login » My Domains
From the My Domains page, click 'view details' for the relevant domain, and then click 'contact information'. From this page you can alter the contact details.
NB - for .uk domains you cannot change the registrant company name this way. To do this, please go to Nominet directly http://www.nominet.org.uk/registrants/maintain/transfer