There's a couple of ways you can do this.

1. With your clip selected, go to the Effects tab, then in the left pane, go to Video Effects. Scroll down to and select Speed. You can move the slider to a value that is <1 to slow down the video. This wil also increase the duration of the clip.

2. Above the timeline, locate the Mouse Mode icon, click on it to reveal the menu, and select Stretching. Now grab the right edge of your clip and start dragging.