How to reset desktop? 16.04 upgrade?

If you can get the right click option on the desktop (to change the wallpaper, et cetra ) you should be able to get a terminal session on the desktop.
try ctrl + alt + T while in the desktop.
if a terminal pops up try these commands

dconf reset -f /org/compiz/

setsid unity

Edit: alteatively, if you caot get a terminal with the keybindings, when you right click on the desktop,
It should have an option to create a new folder.
Do that.
Then click to open that new folder and the file manager will open.
In the file manager go to the side panel area and go to the option Computer.
The main window should show a bunch of folders, locate the folder marked usr.
Then in usr locate the folder share.
Then in share locate the folder applications.
In the applications folder, scroll down till you get to Terminal.
double click and it should open a terminal.
fwiw