The answer to 'how to unclog pipe' is to use a plunger, drain snake, or chemical drain cleaner. First, try using a plunger to dislodge the clog. Firmly place the plunger over the drain opening and plunge up and down vigorously for 1-2 minutes. If that doesn't work, use a drain snake (also called a plumber's snake or auger) to physically break up the clog. Feed the snake into the drain and crank it back and forth to push through the blockage. As a last resort, you can use a chemical drain cleaner, but be sure to follow the product instructions carefully.
If these DIY methods don't clear the clog, it's best to call a professional plumber. Stubborn clogs may require specialized tools or plumbing work that is best left to the experts.