6 Months ago I brought home an underweight, terrified border collie, who was afraid of everyone and everything, literally, she was afraid of roast beef the first night.
She would charge, growling at people and snapping at them keep them away.
She growled at everyone, except her 2 people.
Fast forward to now.
She meets most people with enthusiasm, instead of charging at people growling, she is running to them looking for treats, and affection.
Strangers can be tolerated, and will usually become friends.
Find the right trainer, read as much as you can, and spend time with your dog.
Wasn't easy, but good things rarely are.