Always focus on volume but not just volume. Focus on recruiting the entire chest as one instead of utilizing specific movements thinking they will only increase specific portions of the chest. Look at the big picture per say, and design your chest day to recruit the entire pectoral as a whole using various angles, grip variation, deeper ROM with a direct focus on contraction, ultimately using a wider range of movements targeting all fibers. The upper fibers will grow over time, following all fibers below.