You can easily change which of your characters appears in a particular panel by following these steps:
Click on the page you want to edit
Click on the panel image that you want to edit
Click on the Generate tab
Locate the People, Places and Objects icons and select People. This will show you which of your personalized characters are currently assigned to that panel.
Make sure the characters you want to appear in the panel are selected, and deselect any characters who should not be included in that particular scene. Then review the written prompt and make any adjustments necessary to ensure that all the details match up properly.
Most errors that the AI makes with character generation will be caused by a mismatch between the characters that are selected in this menu and the characters that are detailed out in the written prompt.
Next click on Generate image and waited for the newly generated image to appear (this can take several minutes). Then select the new image and click the save button to replace it in you book.