It's pretty common for dogs to show aggression during walks when they are on leash. It's not natural for them to be on a leash so this can lead to unnatural behavior. Some of the nicest dogs off leash can get very aggressive at the slightest thing when they are on leash. Obviously for safety reasons, keeping your dog on leash most of the time is always a smart thing to do. Here are some tips to get your dog to stop acting aggressive during dog walks.
1- "Good Boy"
Or "good girl". Once you notice a dog or anything else that causes your dog to act aggressive on leash, keep walking normal but start praising your dog.
If your dog begins to act aggressively tell them no and get in front of them to calm them down.
3- Turn Around
If your dog still is acting aggressive turn around and go the other way.
This should start to work pretty quickly. Probably not the first few times, but within a week you should notice real progress.