You will need to add weight lifting into your program. As Sue mentioned, heavier lifting will promote muscle building. You can do HIIT a couple days/week and then add some heavier lifting 2 more days into your weekly routine. Without knowing anymore information about you it’s almost impossible to give you specific answers. Get with a nutritionist or a dietitian and let him/her work on your diet to help you with your goals. I would also suggest to get with a certified Personal Trainer to help you with a more specific and individualized exercise program. All you are going to get from this forum are generalized answers. Hiring a trainer even for a few sessions could make all the difference.