Depends on what's causing the barking. Hiring a trainer can help further (you can ask your vet about different trainers they recommend).
If it's out of boredom then the dog needs more interaction from you in the form of more physical exercise and mental stimulation.
If it's out of nuisance alert barking (at basically the sight and sound of anything around it) then you need to remove the dog's access to that stimulation when you're away and can't correct it. If they're outdoors, bring them inside and gate them away from the front room of the house so they don't have access to windows looking out to the street.
If it's out of loneliness then you as the owner need to either consider either working from home, coming home at your break times to spend time with them, taking them to work with you if possible, hiring a dog walker to come by your home once or twice depending on how long you're away, using a day pet sitter or a family member/friend who you trust and is also home, using a doggy daycare, or thinking long and hard about what's best for the dog and think about surrendering them to a no-kill shelter/rescue or finding them a new home.