Train your dog to react differently to certain stimuli. So instead of your dog barking at the doorbell, teach them to have a positive association with it. During training anytime they hear the sound, offer them a treat, or teach them an alternative command, such as sit, or go to their bed, etc.
That way when a guest arrives and rings the doorbell, the dog associates it as a positive experience and won’t react by nuisance barking. This is also a good habit to teach, if your dog is easily excitable or likes to jump.
Training can be a challenging task, especially in older dogs who have previously acted poorly to certain situations. Remain consistent, and don’t give up, all dogs can learn new tricks.
The earlier you teach them good behaviours the better – the better for you, your guests, neighbours and your dog themselves.