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.
- Go to Setup > Product Configuration
- You'll land on the Message Catalog Editor tab
- Search the beginning part of the message "If you donate and have not already registered"
- You'll find the specific message that is appearing on your donation form
- If you want to edit the text, click Edit Custom. Enter your desired text and click Submit.