Ok thanks, I have wrote this code :
but the device answer me :
<rci_reply version="1.1"><set_setting><user><error id="4"><desc>Field specified does not exist</desc><hint>cleartextpassword</hint></error></user></set_setting></rci_reply>
I think <cleartextpassword> replace <password> but without encryption, but maybe I'm wrong. The <cleartextpassword> field is accessible with all version on device and all firmwares ? I have just 2 users (Root + thomas), and my firmware version of the device is :
Firmware: 188.8.131.52 (Version 82000856_F6 07/21/2006)
POST: 1.1.3 (release_82000867_G). If I don't specify the password in the RCI command, this command is accepted by the device and the name is changed in the users management tab, but the password is not kept.