I want to change the board support to my own board but not sure about how to do it. I took a nRF51_SDK_9 example, for instace IAR Embedded/nRF51_SDK_9/examples/ble_peripheral/ble_app_beacon/
What you circled is a location where you can input defines that are you at compile time. If you inserted in your code a define like MY_BOARD
that makes your project be compiled for your board, you can simply define it there.