In PianistHD: Piano +, "Note Naming" is decided when you first time open the app. However, you still can change it after using it for a while by doing these following steps: 

Step 1: 

Choose the Setting icon (on the right side)

Step 2: 

Scroll down and click "Advanced Settings"

Step 3: 

Choose "Advanced Keyboard Options" and change it in "Note name".