To overhaul something means to take it apart completely and fix or improve every part of it. When mechanics overhaul an engine, they disassemble the entire thing, inspect each component, replace worn parts, clean everything, and rebuild it so it works like new or better than new.

The word suggests thoroughness and effort. A school might overhaul its math curriculum by rethinking how every grade teaches numbers, redesigning the entire approach rather than tweaking a few lessons. A ship in for an overhaul gets stripped down to bare metal in a shipyard for months of intensive work. When your parents overhaul the garage, they empty it completely, sort through everything, and reorganize it from scratch.

You can use overhaul as a noun too: “The old bicycle needed a complete overhaul” means it needed serious repair work, not just air in the tires.

The word implies that something wasn't working well anymore and needed more than a quick fix. An overhaul takes time, skill, and determination, but the result is something that works properly again, ready for years of reliable service.