Managing media with the library
Update imported files in the library
If you use an external editor to modify files that you have imported into Flash, such as bitmaps or sound files, you can update the files in Flash without reimporting them. You can also update symbols that you have imported from external Flash documents. Updating an imported file replaces its contents with the contents of the external file.
1 Select the imported file in the Library panel.
2 Select Update from the Panel menu for the Library panel.
This is very useful option. Imagine You have a hundred images generated automatically by another software. You have imported them into Adobe Flash CS3 for example for a frame by frame animation. You create Your animation and thought that images should be brighter...
Now You do not have to remove each imported item separately and them add it again, You may just select imported items and update them all at once. 
