Plug both MP3 players into USB ports on your PC. In 'My Computer', double-click on the icon for one of the players, to open a window displaying the contents. If the window opens full-screen, click the 'restore' button (at the top right of your screen, between 'minimize' and 'close') to make it smaller. If necessary, drag the edges of the window so that it only fills part of the screen.
Repeat for the other MP3 player.
You've now got two windows open, side by side. In the window for the old player, go to 'Edit' and click on 'Select All'. Click on any file or folder within that window and (while holding the mouse button down) drag it across to the other window.
Job done.
Chris