When you have not scanned the horse microchip (click 3 times quickly on the button), you have to manually associate the horse with the training.
You can change the horse associated to a training by 2 means:
Before synchronizing the trainings to the cloud, in the Trainings tab:
Sometimes, the horse microchip can not be detected or the horse doesn't have any. In these scenario, the App will ask you to manually associate the training with a horse before synchronizing it to the cloud. The training appears in red.
While reviewing your trainings once they are synchronized, in the Trainings tab
If you did a mistake and want to change the horse associated to a training. You can simply go to the Trainings tab. Select the training you would like to modify, then, click on the "Edit Training" button. You can now edit the horse by selecting another one from your list on Change Horse.