The threshold height of a door is the bottom portion that extends across the width of the door. The threshold helps to seal the door and prevent drafts, preventing cool or heat from escaping. In order to adjust the threshold height, you will need to first remove the existing threshold. To do this, use a screwdriver or drill to remove the screws that are holding it in place. Once the screws are removed, you can then lift out the old threshold and measure it against the new one. If the new threshold is higher or lower than the old one, you will need to make adjustments to the door frame accordingly. For instance, if the threshold is higher, you will need to add shims to raise the door frame. Once the frame is raised to the correct height, you can then install the new threshold and screw it into place.