Twist or pull your threaded stopper into the up position. Some models of threaded stopper twist open and close. Other models push in and out.
Look for a screw on the side of the stopper just below the head. If the stopper contains no screw, pinch the stopper shaft with needle-nose pliers. Grab the knob atop the stopper with a second pair of pliers.
Loosen the screw on the side of the stopper with a Phillips screwdriver. Otherwise, hold the shaft in place with the needle pliers while loosening the knob using the second pair of pliers. With the knob removed, turn the shaft counter-clockwise with a slotted screwdriver.
Lift the stopper from the drain.
Wipe the stopper shaft down with a rag. Remove any soap scum or hair clinging to the center plug.
Apply distilled vinegar to the rag, and wipe the plug thoroughly. Vinegar dissolves mineral buildup caused by hard water. To remove buildup, swap the rag for a wire brush.
Reassemble the stopper. If the stopper contains a screw, reinstall the stopper first before tightening the screw. Tightening the set screw secures the stopper to the flange inside the drain opening.