An FLA file is a Flash source file that contains all the animation and graphics. To manage this file you should have Macromedia Flash MX 6 or higher installed on your computer.
To open an FLA file in Macromedia Flash editor you should select "File/Open" from the top menu and browse to the location of your FLA file. A simpler way to open your file is just to drag and drop it from your file browser to your Macromedia Flash editor window. Also, if FLA files on your computer are based on the Macromedia Flash software, you can just double click it in your file browser and this file will be opened and ready to use.
Cheers, Tom 😉