change lower cable of IRB6400

Are you talking about the belly pack? The power and resolver cable that runs to each motor? If so there isn't a upper and lower cable pack. It's all one pack. You have to replace the whole robot at once on a 6640. Best thing to do is lay the robot out as flat as possible (if possible) and start. Pull the covers off each motor and sstart pullig the cables through the robot.

For your IRB6400, you need to move axis 2 forward and axis 3 down so the counter weight is up and out of the way. Axis 1 should be near 0 deg.
Pull the screws on the SMB unit and only remove the R2.X2 connector.
Pull the screws on the Brake Unit (has brake release buttons) so it can be accessed.
Remove axis 1 cover (toilet seat) to access and disconnect connections R2.MP1 at axis 1 motor, R2.MP2, R2.MP3, R2.MP4, R2.MP5-6 and R3.BU1-6 on the Brake Unit.
Remove the screws to the cable plate and the access plate on the opposite side.
Disconnect the ground wire and remove the nuts holding the cable in place. It's not easy if the access to the rear of the robot is blocked
Note how the cable track is situated before pulling the lower harness out so you can make sure the new one is folded the same way.
Pull the cable harness out.
Inspect and wipe down the nylon glide with a clean cloth and remove any debris/grease from the cavity.
Reverse the process to install.