how download my music in itunes

If they are iTunes purchases then you might be able to redownload them via the Purchased tab in the iTunes store app on your phone (whether you can redownload music depends upon your country, if the music is still in the store, and whether you've moved countries since buying it).

If you can't redownload it then you can sync your music from your computer's iTunes library (assuming that you have a copy of it in your library)

Syncing with a computer : Sync your iPhone, iPad, and iPod with iTunes using USB - Apple Support

If you've subscribed to iTunes Match and have uploaded your own music to it, to use it on your phone : iTunes Match on iPhone, iPad, or iPod touch - Apple Support