Before anything else, it is important to check that the tools used are adequate to your dog’s size and to the length of its coat. Using inadequate tools could irritate your dog’s skin and make brushing unpleasant.
It is recommended to handle your dog from a very young age in order to teach him a series of practices, including brushing. If at adult age your dog does not want to be brushed, it is best to take it slow. You can start with a small area, preferably an area where it likes to be stroked. Think of reassuring it and, more importantly, reward it. It needs to associate brushing with a pleasant activity! Day after day, you will be able to progressively increase the brushed surface and brushing time. You can use a detangling and polishing spray to help.
If despite all of these precautions your dog let will not let itself be handled, it may be useful to see a veterinarian specialized in behaviour.