You must first create a LIDS free session. Then set your password using the "-P" option just like you did the first time (you will not be prompted for your current password). After resetting your LIDS password, you must tell LIDS to reload its configuration files.
WARNING: It is possible for someone with root access to obtain the LIDS password if /dev/pts is mounted. In order to prevent this, you can either unmount /dev/pts or protect it.