My harmonica sounds out of tune. What should I do?

If your harmonica is out of tune, you could either:

1) Retune the flat reeds yourself - This is a difficult to do, and requires practice.

2) Buy a new harmonica - This is your easiest (although not your cheapest) option.
Which Harmonica to Buy? The Best & Worst Harmonicas
https://www.harmonica.com/which-harmonica-to-buy/

Harmonicas can sometimes go out of tune in a few years of playing because of reed fatigue.  How old are your harmonicas?  If yours have gone flat sooner than expected, you might be playing them too hard.  Playing softly will extend the life of the reeds.

Also see this article: 
How do I prolong the life of my Harmonica? (+Care & Cleaning Tips)
https://support.harmonica.com/article/85-how-do-i-prolong-the-life-of-my-harmonica-care-cleaning-of-harmonica-tips