When in the media editor, click on the Text Icon option to edit text and review available text effects for the vibe:
After clicking into the Text section, you can click on the existing text to open the text box. From there, you can either click the 'X' on the text box or 'Delete' underneath the Flyer Media Image, to remove sample text:
You can then type your own text, select your desired text effect and text color and view the update on your Flyer Media Image: