Log in your Seller Assistant account. Click on your icon on the right hand side and choose "Profile".
Write the new email you want to use in the email bar and press "Save changes" button.
Verify your email as you will receive a verification letter on your new email.
After that, you can log in to your account using your new email address. All other information in your account will remain unchanged.
You might be also interested to read an article How to reset password in Seller Assistant.