You throw the ball, your dog races after it, then stands five steps away with the prize in their mouth. Fetch has several separate skills: chasing, returning, and giving up the toy. Your dog may know only the first one. Do not chase them around the park or pry the ball from their mouth. That can make possession more exciting or more worrying.
In a safe enclosed area, begin with a low-value toy. Offer a treat or another toy they like better. When they release the first toy, reward and sometimes hand it back. Add a "drop" cue only when the exchange is easy. Separately practice rewarding your dog for moving toward you while holding a toy. Build the distance little by little, then add a gentle toss. If the ball makes them stiffen or growl, stop the game and seek qualified, reward-based behavior help for guarding rather than attempting more swaps over the prized item.
Keep the game safe
Use a suitable ball that cannot be swallowed or lodged in the throat, and watch for fatigue or injury. If your dog will not return reliably, play only in a secure area or use an appropriate long line attached to a harness where it cannot tangle, away from roads and other people. End the game before your dog becomes frantic, and make sure "drop" often leads to more fun, not always the car ride home.
Fetch can be a good game, but it is not a compulsory exercise. If your dog likes carrying a ball more than retrieving it, a calmer sniff walk or another play style may suit them better. For more ways to meet an active dog's needs without relying on one ball game, see the high-energy dog play guide, don't force a trade if the dog guards it
Sources: American Kennel Club: How to teach your dog to fetch. American Kennel Club: How to teach your dog to drop it.