您需要先删除约束，然后重新创建它.您的第二次尝试是正确的，但您需要删除 和.

You need to drop constraint first, and then recreate it. Your second attempt was right, but you needed to remove and.

alter table AllowedCars 
  drop constraint FK_AllowedCars_CarID 

alter table AllowedCars 
  add constraint FK_AllowedCars_CarID 
      foreign key (CarID) 
      references Cars(CarID) 
      on delete cascade 
      on update cascade