Here are the steps to replacing the PTFE tubing inside of your gMax's E3D hotend.
You will want to start by removing the LED light cover from your extruder.
Next you will need to remove the fan shroud to be able to loosen the bolts holding the E3D in place. You can remove the BLTouch as well if you would like a little more room.
With dual extruders, you will want to remove the active cooling fan to be able to access the E3D mounting screws. It is recommended to unplug the fan from the quick disconnect when removing it. Otherwise you run the risk of jerking the wires out of the active cooling fan and will need to replace it.
Once the E3D is off of the extruder, you will be able to remove the PTFE tubing. You will need to hold down the black ring around the tubing when removing. This ring is used to prevent the tubing from coming out the E3D. You can then insert the replacement tubing and remount the E3D. And just reattach any piece of the extruder that was removed.
Once the extruder is reassembled, you will need to re-level the bed and you will need to dial in the z-offset of the BLTouch.