From Android Wiki
adb sync is a tool which may be used to copy files from the computer to the device if the files has been changed. By default the command will synchronize /data and /system unless one option is specified. For the command to operate properly a system variable must be created and defined. ANDROID_PRODUCT_OUT must designate the full actual path to the directory containing a file structure with system and/or data subfolders containing the files and folders changed. In windows this can be created by going to system properties, advanced properties, environment variables. After the variable is created the system must be rebooted. If adb has been added to PATH then adb sync may be executed from the run command.