You can reset your password by receiving a password reset email.
After multiple failed attempts to sign in to Niconico, you will be temporarily locked out from trying to sign in for security purposes. This lock lasts certain amount of time and will then clear on its own.
Enter your account email address and click on "Change Password"
Please be sure to type in the correct email address.
No blanks, half-width numbers and alphabets only.
If you have a mobile phone email address associated with your account, please enter your other email address.
You'll receive an email with a link to reset your password.
The password reset email will typically arrive within a few minutes but sometimes it can take a while.You should get an email within the next 24 hours.
Make sure to check your spam/junk folders. Sometimes these emails get sorted automatically into the spam folder based on your email settings.