How to retrain existing SSD_Mobilenet_MSCOCO model for a new class label

Dear all,

I run the trained model (ssd_mobilenet_v1_coco_11_06_2017) to detect bounding box on my dataset, but this trained model is not performing well because the main object that I am detecting is not in trained model.
Here is example that I am following:
https://github.com/tensorflow/models/blob/master/object_detection/object_detection_tutorial.ipynb

First Question: How can I retrain the existing ssd_mobilenet_v1_coco_11_06_2017 model with one addition class. For the new class, I generate boundingbox annotation in xml format.

Second Question: How can I train a new model only with my own dataset which has only two classes. If possible share any demo or code or tutorial.

Thank you.

-Humayun