If you push the part where the plastic pipe goes into , towards the main body of the fitting the plastic pipe will pull out . Hard to describe easy to do .
Take the fitting to a pneumatics stockist they will have one . The plastic pipe is measured on the o/d. E.g. 4mm the threaded part will be BSP E.g. 3/8 inch bsp (British standard pipe )
Just take the old fitting they will match it up 
Use some ptfe tape or some pipe sealant on the thread when refitting .