To Switch Between Paper Space and Model Space on a Layout

When you're in paper space, you can double-click within a layout viewport to access model space.
If you are in model space and want to switch to another layout viewport, double-click within another layout viewport, or press Ctrl+R to cycle through the existing layout viewports.
To return to paper space, double-click anywhere outside the layout viewport.

NOTE:To prevent changes to scaling, lock the layout viewport display before you access model space. When the display is locked, you cannot use ZOOM while you work in model space.