So, that's the well known Year2029 Bug.
Oh. I see. It will even repeat from now on in ten years, every day.
(Not like the Year 2000-Bug.)

But anyways, you don't write what you are trying to do,
so here comes some general advice.

And I don't know, which level you are in programming,
so my 2 cents are even more unspecified.

I'd try to accomplish things as simple as possible.
Reading your question, I immediately thought of a regex,
next thought was grep/map.

If you know these tools, you can accomplish much more
than only changing the digits of a date.

And even when you decide other, (hopefully)
and use the date in another format,
you've learned something valuable.

Best wishes, Michael