Most overlock stitch sewing machines have two needles that move up and down together. You can sew with only the left needle for a wide stitch, only the right needle for a narrow stitch or with both needles for a sturdy double-needle stitch. The needles are a bit tricky to replace because the screws which hold them in place are small and placed very close together. An overlock stitch machine is sometimes called a serger.
Loosen the screw located directly over the needle with a tiny sewing machine screwdriver. If you do not have the screwdriver that came with your machine, you may use the screwdriver from an eyeglass repair kit.
Pull the needle straight down 1/2 inch. Note the position of the flat side of the top or shaft of the needle. It will usually be on the back of the needle.
Remove the needle.
Position the new needle with the flat side of the shaft in the correct position.
Insert the needle into the machine by pushing it up as far as it will go.
Hold the needle in place with one hand while you tighten the screw with the other hand.
Replace the other screw in the same way, if necessary.
For best results replace your needle frequently. A few types of machines are designed so that it is easiest to remove both screws before you remove the needles.