From the main RPC log in page, click the "Forgot your password" link (screenshot below)
Enter the email address associated with your account
Check your email for a link to reset your password (sometimes this can take up to a couple of minutes to receive - peek in your spam folder if you're not seeing it!)
If you don't receive the email, you may have used a different email address to set up your RPC account. Try entering another email. And you can always update the email associated with your account in your profile after logging in!