Go to the Windows Start menu. Click All Programs > ANSYS 19.2 > Help > ANSYS Help.  A window will open.

Along the bottom of that window are a list of alphabetic ranges.  Click the IN-OP link.

One of the buttons that appears above the list is Mechanical APDL, click that.

Now there is a list of Documentation, look in that list for Structural Analysis Guide.

Now there is a list of chapters. Click 7. Buckling Analysis.