To edit the content under "My Story" on your fundraiser project, log in to your fundraising 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 The Story section. Upload a new image and when done, click on "Save changes".
To update the story copy, click on "Edit story" below the The Story section. Edit the story copy and when done, click on "Save changes".