While it may be theoretically possible to convert an automatic lawn mower to a manual mower, it would require extensive modification and could be dangerous. You would need to remove the engine and all the associated components, such as the fuel tank, carburetor, spark plug, and starter. You would also need to install a new manual drive system, including a set of wheels, a handle, and a transmission. This process would likely require specialized tools and skills, and could be time-consuming and expensive.
A safer and more practical option would be to purchase a dedicated manual lawn mower. Manual lawn mowers are designed to be lightweight, easy-to-use, and low maintenance, and they are available at most hardware stores for a relatively low cost.