This is considered as a custom code modification

That mean I can change it by edit the code? If yes, is it hard or easy to do.

Yes, correct, by changing the code. I believe it is easy if you have enough knowledge in PHP

Can you show me how to change it? If not that ok.

This is General Questions section.

Topic was moved from General Questions.

You can find the username validation function in this file: ow_system_plugins/base/controllers/join.php

But, we strongly do not recommend to allow spaces in the usernames since the usernames are used in the URLs and the space may cause the problems with the software routing system, thus, lead to negative consequences.