Nominet is an organisation that oversees and looks after all of the .UK domain names. If you would like to change your IPS Tag via Nominet directly, you can do so here.
For first time users with Nominet, choose the option First time logging in or forgotten your password?. You must enter the email address that was used to register your domain name so they can ascertain that you are the genuine owner. Follow through the registration procedure until you can login to the Nominet control panel.
Once logged into Nominet, you will see on the control panel an option to change the IPS Tag for your domain name. Please note, Nominent may charge a fee for processing this request. The IPS tag should be as per the hosting company that you wish to host your website with. HostPresto!'s IPS tag is 'ENIXLTD'.