Shrinking SQL Server database
You can shrink you database file using SQL Server Management Studio.


  
  In Object Explorer, connect to an instance of the SQL Server Database Engine and then expand that instance.
  Expand Databases and then right-click the database that you want to shrink.
  Point to Tasks, point to Shrink, and then click Files. 
  


Source: https://docs.microsoft.com/en-us/sql/relational-databases/databases/shrink-a-file

Please be advised, that the databse needs at least some free space to work. So don't shrink it all the way down to 20 GiB