First I suggest getting a blood test done just to make sure every thing is in order. If she suddenly developed a metabolic problem then it may not be so easy to lose the excess weight. My intact female has weight problems, before she had pups and after ward..she is just a piggy
. I do keep her at an idea weight (but, did manage to put too much on her last heat as we thought she had taken with the AI). Any way...first and foremost just like we humans: Increase activity levels take her for more walks during the day and build her up slowly to longer walks...swimming is great, but, may not be so great for weight loss as the water temperature is usually colder than body temperature and signals the body to store fat for insulation...we humans should keep this in mind as well...
. I am not completely for senior diets/or weight loss kibbles for dogs, I believe it was Mordy who mentioned that weight loss kibbles actually have higher Carb content - which is exactly what you don't want.
I would go with a home prepared raw diet to help lose weight...this really helped my porky Rottweiler and Newf when I had to get weight off them. I used lean cuts of meat and lean hamburg, ground Turkey, heart,liver (make sure all organ meats are organic), and ground egg shells for calcium...vegetables, oats, wheat bran or ground flax seeds, low fat yogurt or uncreamed cottage cheese. Some oil of your choice I like Olive oil, Flax oil, Hemp Seed oil.
You can mix this up and use what ever lean meats you want...I had wonderful success with weight loss and natural diets. After my dogs got to their ideal weight I started feeding kibble again...but, still feed natural home prepared meals as often as I can (due to storage space...much cheaper to buy in bulk).
You might need to include a daily vitamin/mineral supplement as well.
As Renee mentioned EVO is a good food to put your over weight dog on also...it will help your dog loose weight as you don't need to feed as much. For snacks try only to give lean cuts of meat, or lean hamburg balls perhaps rolled with oats or flax seeds (crushed).