pyqgis - How to rollback the committed changes in the Vector Layer in qgis

I selected few features in the vector layer and updated the field. But I want to rollback the changes committed to the vector layer. How can I do that in QGIS?

I have used the following coed:

layer.rollBack()

But it is of no use. How can I rollback the committed changes in the vector layer in QGIS?