Comment Cleaner is a WordPress plugin that allows you to delete old comment data based on user-defined settings. You can specify the age of comments to delete and choose to delete email addresses, names, and website URLs associated with the comments.
Names will be renamed to “Anonymous Guest”. The email address will become “example@example.com” and URLs will be made blank.
Download it from: https://github.com/donnchawp/old-comment-cleaner
Install in the usual way. Please be sure that you have backed up your wp_comments database table before installing this plugin.