How to remove secondary file group
The clustered index structure is the table in SQL Server.
In order to remove a filegroup, you first need to remove all data from the filegroup.  There are several ways of doing that, including using the GUI, like this:

Right-click the table, and choose &quot;Design&quot;
Right-click the &quot;empty&quot; area of the top design panel, and choose &quot;Properties&quot; like this:



In the &quot;Properties&quot; pane (typically shown to the right of the &quot;Design&quot; pane), you open the &quot;Regular Data Space Specification&quot; and change the name of the &quot;Filegroup or Partition Scheme Name&quot; to the name of the target filegroup:


Be aware, this method will result in SSMS generating code that actually drops-and-recreates the table.
An alternate method of doing that would be to rebuild the indexes for the table, specifying the target filegroup in the rebuild command.  I've written a blog-post showing the code I use to accomplish that over at SQLServerScience.com