Contact Wetherys. The current owner and the new owner will both have to sign. Only the original owner that invoked the non-racing agreement can get it revoked.
I just signed a headed price of paper saying that I wouldn't run her in any form of racing, be it point to point or under rules. I got her from North Farm Stud.
Tbh she was such a cr*p racehorse you'd never of bought her with intention of racing anyway