Q: How do I add music to my iPhone6s from my desktop?

This explains how to sync all of your iTunes Library content to an iOS device.

Sync your iPhone, iPad, or iPod touch with iTunes using USB - Apple Support

To summarize....

1. Connect your device to your computer with the cable and launch iTunes (if it does not launch on its own)
2. Select your device in the left side. A blue iPhone type icon shows in the support article under step 4 in the upoer left part of the sidebar. You click on that to select your device
3. Click on the Music heading under Summary and Apps on the left sidebar
4. Select all of the songs, albums, artists, playlists ... Or however you have your library set up ....that you want to sync to the phone. You can sync the netire library if you desire to do so. See step 5.
5. After you have selected all of your music, click on Sync or Apply in the lower right corner of iTunes.

You can likewise with apps, books, movies, etc. if you want lo load any of that content to your device. If iTunes auto syncs your device and you want to make changes, just leave the phone connected to the computer and go back through the various sections of iTunes and select exaclty what you want. Then click on Apply in the lower right corner in order to sync again.

Define up-to-date.

It must be version 12.3 or higher to communicate with your phone.