legally change my name

The first step in making this happen is obtaining an original
birth certificate, as I was born in New Zealand I have had to send a request over for it. Once the certificate arrives I can
then progress to get the name changed.

For 12 years I have been using my second name, my first name
my parents ramdomly picked. To me a name should have meaning or be a tribute to someone who influenced the parents.
My second name is that of my grandfathers who is still with us, Apparently my late grandfathers name was not cool in the 70’s when I was born.

So the intention is to drop my given name , replace it with
my second name , and then my middlename will be my late grandfathers name.