Your question is not completely clear, are you asking about changing the voltage on the CAN channel, or just wondering about a CAN message?
The voltage on the CAN High and CAN Low are usually controlled by the CAN transciever chip. Most chips put both CAN Low and CAN High at about 2.5 Volts when no messages are active. When a message is being transmitted, the CAN High will pulse to about 4 Volts and the CAN Low will pulse to about 1 Volt. If you want to change those voltages, you will have to research your CAN transciever, some are changeable in software, and others use external parts, you will need to provide more details on your CAN interface if you want answers here.
If you want to measure the CAN voltages, measure them with respect to ground, not each other.
