How to copy android system files?

Have you experienced copying system files, reboot and nothing happen? Unable to replace system files?

One major problem is you did not mount system folder to Read and Write!

Your phone must be fully ROOTED!
After running Root Explorer and inside the system folder. Remember to click on the MOUNT R/W.
If you do not mount it to Read/Write you will not be able to copy new files.

You should have this error message if you did not mount it. Others File Explorer might just show pop-up message and gone... this is another problem. You might not see the message.

Make sure Root Explorer allow in SuperUser! If not, you will not be able to access system files will full right.

Once copied. Remember to set permission. Normally to "rw-r--r--".

REBOOT! New system file will only executed after reboot.

NOTE:
Make sure you rename existing system file to BAK before copying new file! Else you might end up asking around for original file :)