Pop-up bathroom sink plugs offer more convenience compared to sink plugs that must be removed from the sink each time you want to let water out of the sink. The built-in stopper cannot become lost very easily, but the trade-off is a more difficult process of removing the drain plug from the sink's drain pipe. Depending on how your sink plug is secured, you may have to use your strength as well as the right tools to accomplish the job.
Open the sink's drain and allow any water inside to drain before continuing. Use a towel to dry the sink, especially around the drain and the exposed edge of the drain plug.
Grab the edges of the drain plug with your fingers. Pull up firmly on the plug, wiggling it as you pull to remove it from the drain.
Wrap a thin piece of cloth all the way around the exposed top and edges of the drain plug. Clamp a pair of channel-lock pliers to the edges of the top of the drain plug, making sure the cloth is between the pliers and the drain plug.
Apply pressure to the pliers' handles to turn the drain plug counterclockwise. You may have to apply more pressure at first to break the bonds of plumber's putty securing the plug in the drain.