How do you replace ball valves?

Depends on the valve type and style:

Threaded valves - Remove and replace with the same style, size and length or longer if the piping can be cut and re-threaded.

Welded either socket or butt welded - If installed without unions - Remove the valve by cutting the piping and weld in the replacement valve with suitable length stub ends. If installed with unions - Remove the valve and piping at the unions, welding in replacement new valve, union halves and suitable length of stub piping;