Changing the crankshaft sensor on a 2014 Mercedes Benz E250CDI requires a bit of technical expertise and knowledge of automotive mechanics. Here are the steps you can follow to replace the crankshaft sensor:

1. Locate the crankshaft sensor: The crankshaft sensor is usually located near the bottom of the engine block, at the base of the engine or behind the crankshaft pulley.

2. Disconnect the battery: The first step in changing the crankshaft sensor is to disconnect the battery to prevent any electrical damage or injury.

3. Remove any necessary components: Depending on the location of the crankshaft sensor, you may need to remove some components such as the air intake system, exhaust system, or engine cover.

4. Remove the old sensor: Once you have access to the sensor, unplug the connector and remove the mounting bolts.

5. Install the new sensor: Install the new sensor by aligning it with the mounting holes and tightening the bolts to the manufacturer's recommended torque. Connect the electrical wire to the sensor.

6. Reassemble any components: Reinstall any components that were removed earlier.

7. Reconnect the battery: Reconnect the battery and start the engine to make sure the new crankshaft sensor is functioning correctly.

It is important to note that if you do not have the necessary technical knowledge or experience in automotive repairs, it is best to consult a professional mechanic for assistance.