Change the Nano mode from GUI to headless mode?

After I received the nano board i booted it up first time with the SD Card + monitor and keyboard. I suppose the OEM Config script then configured it as GUI mode. i did setup username password etc and am able to login using the moniter and keyboard connected to the nano.

Now, if i want to go into headless mode where in I can access the Nano over wifi(already set) through ttyACM interface with a ubuntu host… is that possible? OR if the board is setup in GUI mode first time, I cannot change it to headless mode?
If yes, how?
If not, do i buy a new board?

Can someone advise?
Thank you for your time.