How to show username in reply to comment?

the formatting you want to change typically lives in a comments.php file in the root directory of the theme, though different themes can deal with comments in different ways. There are even themes (Divi themes do this, for example) that let you turn elements like author on and off as part of customizing the theme.

If you’re making the changes yourself, though, you’ll need to find the part of the template where the header for the comment is written and add a call to get_comment_author();