To edit the content under "My Story" on your fundraiser project, login to your fundraiser account and view your project page as your donors would see it (My Projects > click on the name of the project).
To update the thumbnail image, click on "Edit thumbnail" below the My Story section. Upload a new image and when done, click on "Save changes".
To update the story copy, click on "Edit story" below the My Story section. Edit the story copy and when done, click on "Save changes".