After you get a nose piercing and have your first nose ring you need to wait a minimum of 4 months before changing the nose ring for a different one.
If you change the nose ring too soon the piercing of the nose could tear and cause infection or bleeding so it's best to wait at least 4 months to allow the nose piercing area to heal fully before changing the nose ring.
If you do want to change the nose piercing ring before 5 months the nose piercing place can do it without risking infection.
So if you want to change the nose ring before 4 months you should have the nose piercing place perform the job for you so that you don't tear the piercing and cause infection.
But after 4 months you can easily and safely change the nose piercing anytime you want too by yourself.