1. Connect the iPhone to the computer with a data cable, open the iTunes software, and click the "iPhone" icon in the upper right corner of iTunes.

2. In the summary interface of iPhone, drag down to find "Manually manage music and videos" under the option. Click the "Apply" button in the lower right corner.

3. Next, the software will prompt that the iPhone has been synchronized with other iTunes, but the iPhone can only be synchronized with one iTunes at a time, so ask us whether to "erase and synchronize" the original database. Click Erase and Sync to continue.

4. After the application is completed, click the "On this iPhone" option at the top right of the iPhone management page, and select "Music" in the left pane of the software.

5. Next, open the music folder on our computer and select the music you want to synchronize. Drag music to the right pane of iTunes software manually with the mouse, and then click the "Sync" button in the lower right corner of the software.

6. Finally, wait for iTunes to synchronize music files with iPhone. Video files can be added manually and synchronized to iPhone in the same way.