Converting multipatch layer into polygon layer using ArcGIS Desktop?

If you have a 3D Analyst license, there is a tool in the 3D Analyst Toolbox called "Multipatch Footprint". Using this tool will

Create polygon footprints representing the two-dimensional area occupied by a multipatch feature class

See tool help at Multipatch Footprint - ArcGIS Desktop Help

You can find the Multipatch Footprint tool in ArcToolbox > 3D Analyst > Conversion > From Feature Class > MultiPatch Footprint

You just point to your Multipatch file as in put and specify an output to save the polygons to.