How to change the words "no preference" to "Please select"?

Hi there, as you can see on my page here http://www.jamjosandbox.com/rubiconcentre/room-booking/ for appointments+ I have 4 service providers which is my Step 2 on my bookings page. You can see it says no preference in the dropdown box. I would like to change this to say "Please Select" rather than "No Preference" where do I change this and is there a way to change this label in order to protect it against updates and so on - maybe a function or override somehow? Thanks for your time on this.