While there are some interior pipe lining solutions, they are just kind of a short term bandaid that will cost more in the long run and has its own complications, the only real answer is to bury brand new pipe. And that can get expensive if a lot of stuff has been built on top and around it which is why most municipalities just live with even major leaks and kick the can down the road until it breaches the surface and starts flooding areas. It is probably cheaper to replace them sooner rather than later in the long run, but no politician cares about how well a city's finances will be decades after they are gone.