Ok I get it !! I found the right process on the web. Unfortunately I can copy/paste the link here.  To switch from unmanaged to managed mode you have to  press the "Mode" button on the front panel 4 or 5 times ! that's the key.  In the manual they said "press the button for more than 7 seconds" but it doesn't work.  thank you Daniel for your help and hope this post help.
Hi all,  i'am trying to switch from unmanagement to management mode on a 2824 but It dosen't work. I pressed the button mode on the front panel for more than 7 seconds, then the switch has rebooted. During the reboot I had the green light mode Flashing. And when the switch has finished to reboot, It was in an unmanagement mode again :/ Do you have any idea why It starts alway with the unmanagement mode ?  thank you for your help. 
unmanagement mode on 2824