You can't. You would have to make a completely new PSN account.
People have been asking for this option for years, and even with the release of the PS4 its mentioned in the PS4 FAQ that you still are not allowed to make changes to your ID once its made. I dont think we'll ever see this option unfortunately.
Come check out my Twitch TV channel, will be streaming various PS4 games from time to time http://www.twitch.tv/reddaxel
One reason it won't happen is because too many bad people will use it as a means to troll people that they don't like. Also, I believe that if we decided we didn't like the name and deleted the account, that name will never be available to anyone even if the account no longer exists.
What I always tell people who have just bought a PS3 and who are going to create a PSN account, pick your name wisely. Once done, it cannot be changed and at a later date, may not have been the name we should have chosen.
Thoughts on picking a name:
Do we want the name to be easily pronouncable?
Do we want it to be remembered and recognized?
Do we want an original name? Something not based on a popular game or movie character? There are a lot of "Solid_Snakes" out there.
There's also tons of "Babygirls".
Do we wish or name to make a statement about ourselves or our personal beliefs?
Those are just somethings. There's many more I'm sure.
We should share this with our friends who wish to join us in cyberspace...
People already abuse PSN in many ways as is. That isn't a wise excuse not to bring the option to us users wanting to change it.
They can do it. It's possible. Sony just doesn't want to waste money on resources for that at the moment.
Very good point. Im sure they would quickly recoup any losses though if they would charge an account registration fee, or even a fee just to change it for that matter. I don't think money is the reason behind it not happening, but i have been wrong before.
They know implanting this feature would rack in revenue like crazy. From their view, I'm sure if they did do it one day, no one would be able to have your old user name if you did change yours. It would stop a lot of abuse (pretending to be the old you).
Either way, it would come down to Moderation overlooking this pretty much. There are different Moderation teams, but does SCE want the PSN Moderation team focusing more on user name abuse more then normal? Probably not even though that's under their job.
If it did ever happen though, either the get a new forum provider and make it more connected to PSN or you use your old user name only for the forums.
How is Black Flag anyways?
Black flag is great, highly recommend it if you are a fan of the series and the timeline this one follows. Then again i liked AC3 as well and a lot of others didn't, so don't take my word for it.