My chihuahua is to fat?

help my chihuahua is to fat she can like barely Move she cant even jump. and she used to run olot and know she doesn't even walk. i took her to the vet and the only thing they said was that she was to fat but don't know what to do and i know for sho shes not pregnant.she doesn't like to eat dog food i actually Always have to mix her food with chiken or stuff like that for her to eat it. plz tell me wat can i do