You can use the small button at the top left corner, at the intersection of the rulers, to toggle between Model and Paper Spaces.
Note: If the rulers are turned off, this button will not appear. Use Rulers to toggle the ruler display.
There are also workspace tabs at the lower left corner of the screen. By default, each file contains one tab for Paper Space. Click the tabs to switch spaces.
Note: If scroll bars are turned off, these tabs will not appear. Use Scroll Bars to toggle the scroll bar display.