Now You Can Customize Your YouTube Channel URL
If you came up with your YouTube channel name on the fly and it no longer suits your purposes, you now have the chance to change it.
While you can't make up your own specific URL, YouTube will offer several options for it based on factors like the creator's username, display name or website (which can also help creators qualify for a new URL, if they link up and verify their official sites with their channel), as well as any vanity URLS the creator already has in use.
There are few more requirements involved – the channel has to be more than 30 days old and have channel art and an image for the channel icon. And your decision about the URL has to be final, as it can't be altered once it's been set up.