I am trying to use the render frame, but the preview quality is really bad an nothing seems to change it. I have chosen Right Click > Preview Settings > Full Resolution, and then Right Click > Update Image, but the quality remains the same.
1.5.6.1 on Windows
I'm a but puzzled about the title of your message.
We need a bit more info on the image. What type is it? What does the popup for the image info have on this?
If your puzzled by the title, you may not know about render frames.
https://wiki.scribus.net/canvas/Help:Manual_Renderframes