How can I choose which desktop environment to run?

In my Ubuntu 16.04, I installed Lubuntu desktop environment and lxde.

At the beginning, when I booted the OS, I saw several options for desktop environment, e.g. Unity, lxde, Lbuntu's lxde.

Now, when I boot the OS, it doesn't show the DE options, but goes directly to lxde or Lbuntu's lxde (i am not sure which one it is).

How can I choose which DE to run, when booting?

How can I switch to other DE, when I am already in lxde or Lbuntu's lxde? How about switch back?

Thanks.

Logout then click on little unity icon from right corner on login window ( where you enter your password )