Contact an INS+ Within your constabulary an tell them that you want to change you're name and why. If not yet in a constabulary tell the academy member who is doing your PA before you start the test.
Contact a DI+ in your constabulary and tell them what you want to change your name to so they can update all the relevant PNCs. If you are a CSO or PCSO, contact a DI+ in Academy (myself included) and we'll get it sorted for you.
When it's done we'll let you know and then you can change your name in the Arma 3 profile and start using it.