can i change username
I believe you can. Here's our forum host talking about it back when the forum opened:
If you just want to change your username, it looks like it can be done on your own, through editing your profile. When you're logged in, just click on your user name or your user avatar in the top corner of the page to go to your profile page. Clicking "Edit" has a field to change your user name.
I don't know whether that process will change the username shown on old posts or not. If that's important either way, send a message to hungryghost before jumping in.
If you prefer, hungryghost could also delete a user account, and start a new one fresh with a different user name.
(hungryghost gets very busy at times, so be patient waiting for a response.)