Lots of ways ...

There is a volume slider in the track header at the left edge of the timeline. Reduce that and it lowers the volume of everything on that track.

Hover the mouse pointer near the top edge of an event on the timeline and it will change to a little hand with a pointy finger. Drag down and a blue line will appear. The farther you drag this line down the quieter that event will be. This lets you adjust the volume of an entire song without affecting other songs on the track.

Probably better yet is to select one track, the Insert / Envelope / Volume. This will place a blue line through the middle of the entire track. Double-click on this line where you want the volume to change and a dot appears on the line, double-click again just to the right of this first dot and a second one appears. Drag the second dot up or down to raise or lower the volume. You can add as many dots as you wish, spaced wherever you want them, and change the volume as much as you want throughout the entire video.