YouTube displays your currently selected Google Plus account username and avatar, providing access to other linked accounts in the Profile menu. You can change any linked Google Plus account username in your YouTube Account Settings menu. YouTube doesn't support changing your original username but does support linking unlimited Google Plus accounts with your profile. You can change your Google Plus username up to three times every three months.
Create a Google Plus Profile
Linking YouTube to a Google Plus profile makes your YouTube and Google Plus usernames and avatars the same, but it doesn't change your YouTube channel URL, which is permanently set when you create a YouTube account. Visit Google Plus to create an account using any name that doesn't include numbers or special characters. To use only one name, enter it in the First Name field and a period in the Last Name field. Link this account with YouTube by opening your YouTube Account Settings menu and clicking “Connect Channel With Google Plus.”
Change Your Google Plus Username
From your YouTube Account Settings page, click “Edit on Google Plus” to open your Google Plus Profile menu in a new browser tab. Google Plus blocks name changes after the first three in a 90-day period, so the only way to change your YouTube username again is to create another Google Plus profile and link it with YouTube. Change your active YouTube username by clicking the “Profile” menu and selecting an account.