Run

DBCC SHRINKFILE (N'tempdev' , 50) -- this will bring the data-file size to 50% of current size (you will not see the file size is decreasing) below command will do it.

Now run below command to set 50% of the current size

(my initial size was 1000MB, i shrink the file to 50% and now issuing below command to set it)

ALTER DATABASE [tempdb] MODIFY FILE ( NAME = N'tempdev', SIZE = 500MB )

GO

After this command you will see the file size has reduced.