A CAB file is a Windows Cabinet archive. This is similar to a ZIP file, but it holds Windows-specific data. One piece of data it commonly holds is an MSU. An MSU file is a Windows Update file. When your computer runs updates, it is running them off of one or more MSU files. An MSU file often runs off CAB files, so if you have an update-related CAB file, turning it into an MSU is simple.
Open your Start menu, located in the lower left-hand corner of your desktop.
Type "msconfig" into the search bar, then click "msconfig.exe."
Drag and drop the CAB file onto the msconfig window. It will read and output the contents into the same window as the CAB. This will include not only the MSU, but any other files contained within. MSUs often come associated with other files.