You can use quick clip software to add background music to short videos, as shown below:

1. First open the fast clip software on your computer (you need to download and install it in advance), and then click the "local video" option above.

2. Select the video with background music in the pop-up pane, and then click the "Open" button.

3. After adding the video, click "Add Music".

3. Then click the "Local Music" option.

4. Select the background music to add, and then click "Open".

You can choose the length of music in the music department.

6. Then click "Save and Export".