Thanks for reaching out to us!
If you’d like the person to be on a flat color background, you can simply use the Effects tab > Sketch tool to paint a flat color over the background.
If you’d rather the person be on a new background image, first open up the new background, then drag the photo of the person on from the Photo Basket at the bottom of the editor. Click the person and a Photo Properties dialog will appear with an Erase icon. Click that Erase icon and paint around the person to delete the background. We have a brief demo video of this here: https://www.youtube.com/watch?v=ciYtYIWIXOs
If you have any other questions, please let us know!
– Regards, The Ribbet Team