The steps to import music from the computer to the iPhone are as follows:

When the iTunes software is ready on the computer, please connect the iPhone to the computer and open the iTunes software.

When iTunes recognizes iPhone6, please click the phone icon in the window.

Next, please find the "Manually manage music and videos" option in the right window, and then click the "Apply" button.

Then please click the "Music" option under the "My Device" column on the left side of the window.

Next, find the music files you want to download to your iPhone on your computer, select them, and drag them directly to the "Music" column in the iTunes window.

Then open the music on iPhone, and you can see the music previously downloaded from the computer in the song list.

If you want to download more music from the computer in the future, you can directly connect to the computer and drag the music. If the above method fails, please click the "Sync" button in the upper right corner of the iTunes window to synchronize data between them. If that doesn't work, unplug your iPhone, reconnect it, and try again from the beginning.