To access this option, navigate to WP Admin -> PeepSo -> Configuration -> Accounts & Security
Under Profiles panel check Allow username changes toggle.
At the very same panel, there is also an option to Clean up third party registrations.
Some plugins (like WooCommerce, EDD and LearnDash) create user accounts where the username is an e-mail address.
PeepSo uses the usernames to build profile URLs, which can lead to accidental e-mail exposure through site URLs.
Enabling this feature will cause PeepSo to step in during third party user registration and automatically generate a safe username for the new user.