If you can log into your account, then you will first need to visit and log into your Nexon America account. You can log into your account from the main Nexon America website here: https://www.nexon.com/account/en/login
After you have logged into your account, click on "My Account" in the top-right of your web browser and select "Account Settings".
Under the section for Password, click on "Change" to change your password. You may be prompted to answer a Security Question if they are available on your account.
You will be asked to enter your current password one more time, as well as a new password.
If you cannot log into your account at all, you can reset your password from the main Nexon America website here: https://www.nexon.com/account/en/reset-password
You will need to enter in the email address for your Nexon America account. A verification code will be sent to your email address.