Available with Image Analyst license.
Available with Spatial Analyst license.
Reclassify an image after classification
After you classify an image, you may encounter small errors in the classification result. To address these discrepancies, it is often easier to edit the final classified image rather than re-create training sites and classify the dataset again. You can make edits to individual features or objects using the Reclassifier tools. Select a feature in your classmap and reclassify it as any other class in your classification schema. All changes that you make are displayed in the Edits Log section of this pane. You can check on or off any of the edits you have made. If you uncheck an edit, it will not be used in the reclassifier. As you are fixing errors, you will want to see the underlying imagery to verify that the new class makes sense. Press the L key to toggle the transparency of the classified image on and off.
Use the Reclassify an object tool 
to draw a circle on the classified image. The segment from which the circle originates will be changed to the new class. If the circle encompasses all of the segment, the entire segment will change to the new class. If the circle only encompasses part of the segment, only the part of the segment in the circle will change to the newly assigned class.
Use the Reclassify within a region tool 
to draw a polygon on the classified image. The current class within this polygon will be changed to the new class. You can use the Any class to change all the pixels or objects within the region to the new class.