Check your email, you will receive an email with a link to reset your password.
Password reset links sent via email expire 3 hours after they are sent. If you need a new link after your current one has expired, visit https://app.box.com/reset to have another link emailed to you.
If you do not receive an email from Box for this password reset, check to see if your Box account is associated with that specific email address or another email address.