How do I hide or edit the "How did you hear about us?" field at the top of the Registration form?

Under Tools > Online Web Registration > Field Options/Labels, you can set this to Required, Optional, or Hidden.  The values that the customer can choose allow you to track your advertising and marketing effectiveness. You should customize the responses in the drop-down.  Go to Tools > Edit Settings > Drop-down Lists > Source (How did you hear about us?), and add all the venues you use.  Track the results via Reports > Source/Referral Report