National Federations can choose to delegate their rights to the Organising Committees for the entries of late entries only as well as the substitutions and additions on the ground.
On the document "Rights delegation for OCs" you can check which National Federation delegates its rights to the Organising Committee.
If you are the rights. you can go to your show in the FEI Entry System.
Close to the name of the horse, you can click on "subst" and "substitute" to confirm that your have found the good horse.
The new horse is automatically acepted.