There are a number of ways to do this.

The easiest way is to MUTE that whole TOP track. Click the circle with a line diagonally thru it in the box at the left of the track to mute/unmute.

Or, you can use the Composite Envelope (on the top track): Click on an empty spot on the track (not on the clip itself) Choose INSERT/REMOVE ENVELOPE, then choose COMPOSITE LEVEL.

That will show you a line at the upper portiion of the track. Click on the line to give you a small box. Make another small box to one side of the first. Click and DRAG the second box down to the bottom of the track.

Play the event .. you will see the top track fade and the bottom track show up. Using this method you can show the bottom video only for a portion of the timeline, unlike MUTE which, if you have multiple clips on the upper track will stop showing them all.

Note: you can add as many boxes to the line as you want. You can slide a box to the left and right as needed. You can remove un-needed boxes and you can adjust the box position up/down as well.