Stairs receive a lot of heavy traffic during years of use. Stairs are made of several different pieces that carry your weight. The tread of the stairs is the top of the step where you put your foot. The riser is the vertical piece at the front of the tread. Stringers sit underneath the stairs and brace the whole system. You can repair risers and treads with little woodworking experience, but you should have stringers professionally replaced if you notice an issue.
Locate the origin of the stair squeak by walking on all the treads. Look for loose screws on the squeaking stair.
Tighten the screws attaching the tread to the riser using a screwdriver if the tread is squeaking at the front.
Tighten the screws holding the riser to the stringers underneath.
Drill two pilot holes on the tread if the step is still squeaking in the stringer locations. You can find where the stringers are located by looking for the attachment brackets on the tread or going underneath the stairs if possible. Drill two pilot holes at each stringer location. Do not go all the way into the stringer. Nail a 8 or 10d nail into the hole.
Slide a mini-prybar underneath the tread, and pull up under each bracket location to remove the tread from the riser. Place a small block of wood underneath the prybar to prevent damaging the other wood.
Take the stair to your local lumber yard to find the correct wood type. Have the lumber cut at the lumberyard to the same length and width as the old stair.
Drill two pilot holes at a 45-degree angle at each stringer location. Make the holes slightly smaller than the 8 or 10d nails to prevent splitting the wood. Nail the tread onto the stringers.