What makes you think historians are unbiased, or agree on things?

Nobody really thinks they are above bias. But hindsight does help you sort out what things really seemed to matter, what things really seemed to happen, what story one ought to tell. It can take decades for anything like a consensus to settle, and there are some issues that there will probably never be a historical consensus on. Even when a consensus exists, the next generation of historians might argue that the previous generation got it all wrong. This is the lifeblood of history. The idea that history is something that you learn, "get right," then mark on stone tablets forever and ever is a completely false one. Historians today do not think of the Civil War in the same that historians of 20, 40, 60, or 100 years ago did. Historians 20 years from now will probably have a new interpretation. A good historian tries to be objective and tries to ferret out the facts of the matter. But a historian is always a human being, embedded in their own time, seeing the past through their present's eyes, and there is no way around this. A good historian believes that all of the past and present is determined by history, and includes himself or herself in that scheme as well.

As for your other question, "how do we choose which one?" you might ask yourself who the "we" is here. The general public, who gets their historical knowledge from a multitude of sources (some of which are good, some of which are terrible)? Educational curriculums, which are often set by state textbook committees (for a variety of reasons, sometimes explicitly political)? Historians in universities who write books primarily for the consumption of other historians in universities? The editors on Wikipedia? There is no single source of historical knowledge, nor one single audience, nor one single set of interpretations about anything. Dig into any topic in even a superficial way and you will find a plethora of positions.