I figured it out. So even though matlab says that the ground truth object gTruth that you export from groundTruthLabeler is read only, you can extract all of the contents i.e. gTruth.DataSource .LabelDefinitions and .LabelData and save them as variables of the same name in your workspace. Then, go into LabelDefinitions and LabelData and change the name of the label that you mislabeled i.e. sedan to hummer. Then you can use the groundTruth() function to put those three back into a ground truth object.