How To Put On Body Weight?

I am, male, 61 years. Since last 3 years, I have tried to reduce my weight, and succeeded. Before, my weight was 77 kg, and now I am 69 kg. I feel too slim now, and want to gain more weight and stronger muscle. I have been trying to gain more weight for another 5 kg but not succeeded yet. I eat considerably healthy food ( my HDL, LDL, Trygliseride, blood sugar level, uric acid etc are all ok) I do exercise, jogging, running, stretching 5 times a week. What should I do?