I have a lot of letters from my grandfather, who didn't see combat, but wrote a good amount of letters.
I also have a photocopy of a letter from my grandfather's brother, who was killed by a Japanese bomb in the Pacific. His was a bit more interesting, since he sneaked through an uncensored letter (through a friend visiting the States) describing what planes he was maintaining, and various dangers/precautions taken against the Japanese.
Again, I hope this is the right sub for this. Point me to the correct one if I'm wrong!
I'm glad your family has kept them!
There are lots of place that would love to have copies of your letters/the letters themselves.
You can send copies of your letters to the Legacy Project.
I'd also reach out to local colleges/university libraries. Their special collections will often take and preserve materials.
Local/state historical societies might be another option. Wisconsin and Iowa both have WWII letter archiving projects.
Museums (ex. the new Museum of the US Army, or the National WWII Museum) might also be interested in letters.
Well, that's entirely up to you. You can keep them, treasure them as part of your family's history and pass them down.
You could donate them, find either a local or national museum who could be interested in having them in their collection.