This brief overview of the XLTools Unpivot Table add-in shows:
How to convert a crosstab (or two-dimensional, 2D, or pivot table) to a flat list in Excel
How to quickly flatten a complex crosstab table with multi-level headers
Download at https://xltools.net/download/
Suitable for Microsoft Excel 2016, 2013, 2010, 2007, and desktop version of Office 365 (32-bit and 64-bit).