Login to https://manage.press8.com with your username and password. Your username is most likely your email address, and if you don't have your password, just click on the lost password link.
Once logged in, navigate to Features=>Telephone Lines. Click on your extension number (recognized by the last 3 of 7 digits). In the "Forward to:" section, enter the number that you want to forward all calls in the "All calls, without ringing telephone, to number:" field. Then click "Save".
To remove the forward, log back in and just remove that number entered before.