If you would rather have say "Supporter No." then:

• On the Campaign you are running the Statements from, click into the Payment Form that you've linked to this Campaign.  Alternatively, go to the Merchant Facility Tab, click into the Merchant Facility, scroll down to the Payment Forms related list and click into the Payment Form that you'd use for your annual statements or the Payment Txn's are linked to.
• Update the value in the field "Membership Id Label" to be what you would like - e.g. "Supporter No."

If the changing of the label is not what you are after, then you can replace the entire heading.  See How to put your company letterhead on the Receipt Template