Once a set of images have been focus merged, Photo will display the Sources panel populated with a list containing the final merged result and all the individual images used to create the focus merge. This gives you the option of re-editing areas of the result using the Clone Brush Tool.
To edit a merged result using the Sources panel:
Ensure the Clone Brush Tool is selected from the Tools panel.
Click an image from the list in the Sources panel to set it as the current clone source.
You can enable the Toggle source preview eye icon which will allow you to preview each image and find the best source to clone from. When you have chosen the correct source, disable this option in order to edit the merged result again.
Using the realtime preview as a guide, paint over the areas of the focus merged image you wish to replace with the clone source.