If you have not moved the object you can simply take another photograph and add it to the project using the menu item. If you have moved the object, you would need to align the new image as described in the Adding texture underneath an object tutorial.
If you no longer have access to the object or do not want to take any more photographs of it, you can create a synthetic view as described below, edit the silhouette and then re-generate the surface using this silhouette.
See also the Optimising the surface geometry tutorial for other automatic ways of improving the surface which might be just as effective.
Click the button. If you have a texture map generated, you will be prompted whether to save a virtual image of the model to help you with your mask editing. This is generally a good idea, since it is otherwise hard to edit the mask precisely. Choose a folder and filename to save the synthetic image in.
The new synthetic image will appear in the Thumbnail Window with an initial mask generated from the current geometry. Synthetic images are not used for texturing and are indicated as type "silhouette".
Note: Make sure that the whole object is visible in the new image, otherwise your geometry may get clipped — if necessary, delete the new image, zoom out a bit and create it again.