Which part of the valve stem do i need to replace?

Yes, you need to have the whole thing replaced. That appears to be a stem for a tire pressure sensor. In order to fix this, you'll have to have the tire broke down, then pull the sensor off the rim. Then disassemble the sensor and place the guts into a new shell. You could just put in a new sensor, but you'd then have to have it programmed into the tire monitor system.