I am not sure if you can do it remotely, or if you have to physically be in front of the computer. All I can say, is to try it remotely, and then please let me know.
Right-click on the system tray LogMeIn icon, and select TOOLS. One of the tools is to CHANGE THE PASSWORD. Select that, and enter the new password twice. That should do it.
Please, let me know if it could be done remotely or not.
(DISCLAIMER: I am NOT an employee of LMI, nor do I have any financial concern in LMI. I am strictly a HAPPY USER of LMI products. I am using PC version and XP SP2. Other systems or versions may act different than described above.)