As we get older it gets harder to increase bone density. I agree with Natalie on getting your doctor’s clearance and doing weight bearing exercises. Weight training with heavy weights can be helpful, but you’ll want to work up to it, if you haven’t been doing it. Add some balance exercises to prevent falls.
I agree with those above who have suggested weight training, particularly weight training with heavy weights. Bones get stronger when they are challenged, but if you already have a bone density issue, you want to work your way into it. Have you been diagnosed with osteopenia or osteoporosis at the hip?