I would recommend replacing all of them. Strings "die" quite fast, but sometimes there is a misunderstanding by new players that they last a really long time. Unfortunately, they do not.
The "elasticity" changes, and they are harder to keep in tune. They sound dead, and damaged strings can cause fret buzz and be hard to intonate.
Definitely worth putting on a new set. You might take the guitar in to a shop and get a set up as well. One thing I find is the less expensive models are usually quite good guitars after a little tweak here and there. A few passes with a nut file in the right situation can make a huge difference.
...so ever since then, I always hang on to the buckle.
