Prevention of guarding or aggression with toys or food
Give your puppy a toy to play with, then call your puppy over to you & exchange toys. (Make sure the toy exchange is of equal value or greater).
Usually puppies or dogs will drop what they have in their mouth for another toy.
This will show your puppy that giving up something can also give back something.
While your puppy is eating throw a more delicious treat in his food bowl. (do not touch his food or pet him) This will show your pup that the hand is give not take.
Never scold your dog for growling, this will increase the behavior or suppress the behavior to come out later. Growling is a warning. Dogs that are not allowed to growl will bite without warning.