If you only need to change the speed of the video track then a simple thing to do is insert a velocity envelope. This will appear as a thin blue line through the middle of the video track. Pull the line down to slow it down. 100% is normal speed, 0% is still, negative numbers go in reverse, above 100% speeds it up. You can also add points along the line to change the speed from point to point.

If you also need to adjust the audio track at the same time, but don't need to have variable speed, then stretching/squishing is probably a better choice. Select all the events that need to be changed (both audio and video) and type G to group them. Then place the mouse at the end of the last event and drag it as if you were trimming the end, but hold down the Ctrl key at the same time. This will let you stretch to the right (slow down) or squish to the left (speed up) the whole set of clips together. Of course, you can also do this to each clip individually too.