When site members register with their email and make a mistake in their email ( for example .con instead of .com). Is there a way I can change their username ?
But how can you know that there isn’t any person somewhere with the suffix .con ?
Assuming they told me they made a mistake and want me to correct it for them 
I don’t think it’s recommended, and for sure you can’t do it for users who log using Google and Facebook accounts or Wix native log-in, but if you have your own custom log-in from, you may be able to catch the error and replace the user-email based on pre-defined rules.