changing a new piano string or wire is not an easy job, it has to be done by a professional piano technician.
piano string or wire , has their lifespan from time to time. the longer the age the more tension it loss,
there are other reason like loose copper wiring in the bass section, rusty wires, broken wires that will generate unpleasant sound like buzzing, vibrate, ringing or even goes dead.
in some situation, these problem which mentioned above can be fixed. if cannot, dont worry, just change them. noted that everytime when new strings are replaced, it needs to take time to season, maybe a few times tuning to let them stay as new strings are come with new tension, they will go out of tune easily, so do not worry when u heard the piano goes out of tune in a few days after getting a new strings.