For WordPress.com sites, you can select the ‘Lost Password’ link on the login page, or go to:
https://en.wordpress.com/wp-login.php?action=lostpassword
If you need your WordPress.com username, login through a web browser and visit the Me section:
For self-hosted installations of WordPress, in a web browser, go to your site's login page and select the Lost Password link. That will email you your username and a link to reset the site's password!