Standalone installer: install GRASS GIS with the required support packages.
OSGeo4W is an installer for a broad set of open source geospatial software packages including GRASS GIS as well as many other packages (QGIS, GDAL/OGR, and more).
Run the downloaded installer as an administrator and select the grass (stable) or grass-daily (development) packages to install.
This can also be done with the command line, as shown below:
osgeo4w-setup-x86_64.exe -q -k -P grass -s http://download.osgeo.org/osgeo4w/x86_64/
Hint for QGIS users: in order to have GRASS GIS support (also in QGIS-Processing) as well as the “With GRASS” shortcut you need to install the “qgis-grass-plugin*” packages.