Haha everyone's dream.
These are counter-cooperative goals. One requires food to build muscle, the other requires less food to shed weight. However, you can do one and end up with the other (hint: it's not the easy one).
Pick one and stick to it. If you think you have enough muscle and will look good if you were to lean down right now, then do a carb cut (your energy levels at the gym are gonna suck) and do volume training with moderate to heavy weights at the gym to smash calories.
If you don't think you'd look good now and want more muscle, the good news is that you can stuff your face with more food than you're eating now, and lift more weight for more reps - you'll develop more muscle, which will in turn burn more calories in a resting state, leaning you up without touching your diet. Pretty cool hey?