If you are in a group with the Advanced Administrator permissions, then you should have access to the Message Catalog Editor where you will find this text. Follow the steps below to find it.
- Navigate to Setup > Product Configuration
- You'll land on the Message Catalog Editor tab
- Canadian clients, please choose English (CA) from the Language drop down menu on the left hand side of the screen and click Refresh List >>
- In the search field, enter the beginning part of the message "If you respond and have not already registered"
- Click Search
- Find the text with "survey: auto_opt_in_message_center" in the key column
- Under the Actions column, click Edit Custom
- Enter your desired text
- Click Submit