Can someone explain how to install libiconv

You would need to build it from source as it isn’t in recent versions of Debian

Why do you need it?

To compile tvheadend

Why do you need to compile TVHeadend? Which feature you need that is not in the APP Store?

If you really need to do it manually than as Sam wrote you would first need to manually compile libiconv

In APP store is an old version
How can i compile manually libiconv

Follow. Instructions on the libiconv website.

Other than it being an old version, is there a but you are trying to resolve of a feature you want from the later version?

Newer versions seem problematic without actually offering any benefit.

If there’s something you are after in a newer version I could produce some test builds

I don’t remember or having issues with libiconv when building TVH for OSMC. You can check Git to see how it’s done