Editing is basically the same as creating new content, except that your input form is already filled with the existing data.
First off, we will change the type of content element, since this is what is causing errors in the frontend. So click the "Edit" icon of your new content element.
Open the "Type" selector and choose "Text & Images". The following dialog box will appear:
When you change a content element's type, TYPO3 CMS will reload the input form to display different fields. This is why it is asking for confirmation about saving before the reloading happens.
Change the text you entered before and use the "Save and view page" action to save your changes and preview them in the frontend.