Firstly it's worth noting that the Submit Button won't appear in the form body of the editor, although you can see what it looks like and edit it in THEME > UI Elements. On the form however, the Submit Button can be seen on the final "View" version of the form.
By default, the Submit Button will always appear immediately after the last question on the form. If you want to move it back to appear after text or content that comes after the last question, then add a question where you want to move the Submit Button to, and make it a "Hidden" field type. The respondent will never see this question, but it will force the Submit Button to wherever you like.