The guest author can be changed in a post when Quick Edit is enabled in the settings for the plugin.
Look for the box captioned ‘Guest Author’ and select an author from the list. Click on the blue ‘Update’ box in the far right corner.
Similarly, the guest author can be selected for multiple posts when Bulk Edit is enabled. Select as many posts as you like by clicking on the checkboxes. Click on the dropdown box above the list where it says ‘Bulk Actions’ and choose ‘Edit’
Click on apply and look for the small box captioned ‘Guest Author’. Select the guest author that you wish to apply to the selected posts. Once you have selected the author, click on the blue ‘Update’ button in the far right corner.