All Happy Dog Care foods have feeding guidelines on the packaging. You should always start by following the guidelines for your dog's weight. Your vet will be able to tell you your dog's weight.
A dog's digestive system needs time to fully adjust to a new food. For this reason we always recommend introducing a new food over several days to make the change as happy as possible.
The ideal way would be to feed 75% of the old diet with 25% of the recommended feeding amount of the new dog food for a couple of days, followed by 1-2 days at 50/50 and for the last two days feed 25% of the old food with 75% of the new.