You press it apart with a big shop press and special pins, then replace the bearings, pin and rod. It takes careful work and measurement to get it back together right.
Certainly buying a new crank assembly is the easiest option, and often cost effective. My supplier does not sell assembled cranks but the parts to do it are $35 and getting the job done is at least $50. Unless you can press it yourself, it pays to buy another engine.
Steve