How to handle Navigation with Action Bar Drawer

I am using a left hand drawer for my App, and am using a v7.widget toolbar which holds the "hamburger" menu icon.

I am also using a BottomNavigationBar and am switching fragments in the display area beneath the toolbar depending on which tab is selected.

This is all working fine, however within one of the tabs I need to "navigate" from one fragment to another and allow navigation back to the first.

This can obviously be done by switching fragments, but what I need to be able to do is

• replace the hamburger menu with a back arrow when the second fragment is displayed, then
• handle the back navigation to swap the original fragment back, then
• replace the back arrow icon with the hamburger icon.

Can anyone offer any suggestions on how to achieve these three steps ?, in particular reinstating the hamburger icon.

Thanks