If you need to change the center height on your lathe, there are a few things you'll need to do. First, you'll need to remove the old centers from the spindle. Next, you'll need to measure the new centers and find the correct size for your lathe. Finally, you'll need to install the new centers and make sure they're properly aligned.
Removing the old centers is fairly straightforward. Just loosen the setscrews that hold them in place and tap them out with a hammer. Measuring for new centers is a bit more tricky. You'll want to measure from the top of the spindle nose down to where you want the center of your workpiece to be. Once you have that measurement, add half of the diameter of your workpiece. That will give you the proper height for your new centers.
Installing new centers can be a bit tricky as well. You'll want to start by lightly tapping them into place with a hammer. Once they're started, use a center punch to mark their location so you can keep them straight when tightening down the setscrews. When tightening down those setscrews, be careful not go too tight or else you could damage both your workpiece and your lathe spindle itself!