or , read instructions .
Instructions :-
1- Open your internet browser .
2- Go to your Google+ .
3- Go to 'Home --> Profile' .
4- Click on your username .
5- Click 'Change Name' .
6- At the first box (left box),type whatever you want with no space.
7- At the second box (right box),add period (.) .
8- Click 'Save' --> 'Change name' --> 'Proceed anyway'
8- Now,you have YouTube username without space :D