A healthy weight for a dog is typically one where if looking down at the back of the dog you see a noticeable curve inwards where the belly meets the hind quarters. If it seems the hind quarters meet straight with the belly then the dog is overweight.
More exercise will help somewhat but this isn't always the best option for an older dog that may have joint pain. Change of diet is usually the best approach. Controlling your dogs feedings is the absolute best way to ensure that they stay a healthy weight. This Dog Food Calculator is a really helpful way to find out just how much you really should be feeding them.
Some people like to feed their dog one big meal but I think for increasing an overweight dogs metabolism you will probably want to split their calorie consumption into two or three equal meals a day.
What I would feed them is a dog food without corn wheat or soy. If the dog grew up on beef then it is likely they may not take to a change in diet very well so beef is okay depending on the cut and the fat content. Personally if I was feeding my dog beef, I would mix in some steamed brown rice with boiled hamburger. This will be easy on their stomach and it removes a lot of the fat that you don't want them to eat.
For treats I would give them low or zero calorie snacks like pigs ears, raw hides and dental chews. I would also consider giving them a boiled egg as it will help with their joints and for a nice shiny coat of fur.
Remember it is normal for a dog to get a bit of nausea or diahreah when changing their diet. They do not have as adaptive and complex of a digestive system as humans do, but this is mostly temporary.