工具栏 - 只需一个活动即可从抽屉切换到后退按钮

I've been searching for a while on how to change between the drawer open/close icon (going from a hamburger to the arrow) to a simple back arrow. My application at the moment only has one Activity which switches between several fragments. At one point, I want to transition between one of the main fragments (ie, one of the fragments in the drawer) to a fragment that hierarchically is under the previous fragment (ie, an "Add New " fragment). In this new fragment, I want to have the Toolbar to show the back button instead of the drawer button.I've been searching for a while on how to chang