How can I make my Bichon less aggressive?

Bichons are usually gentle; being very aggressive may come from lack of security, physical pain, or no proper training. Have the vet check for disease or injury.

They also need enough exercise and play to burn energy and build confidence and happiness, which reduces hostility.

Do proper socialization training with simple commands, and let it meet different people and animals to lower hostility toward strangers and other dogs. If self-training fails, get a professional trainer.