To edit the selection in a new application's window, do the following :
Step 1 - Select part
At first, select the part of the MP3 file you want to edit in a new window.
You can do this by selecting the part of the MP3 file's waveform.
Alternatively, if you know the start and end time or duration of the desired part, enter the start time, end time or duration in the appropriate text boxes.
Step 2 - Edit Selection in new Window
Now, select "Edit Selection in new Window" from the "Edit" menu.