Log in to your NCBI account and click on your username at the top right of the browser page. This will take you to your NCBI Account Settings page. Use the Change button in the Email box to enter a new address.
You will receive an email confirmation message and you need to click on the link in the message within 72 hours to confirm your email. You can always check your NCBI Account Settings page if your email address is confirmed. If you do not see Confirmed next to your email address, you will see a Request a confirmation email hyperlink that will allow you to generate a new request.