You can change the speed of your video using ​setpts video filter.

To speed up your video, you can type:

ffmpeg -i input.avi -vf "setpts=0.5*PTS" output.avi

Note that this method will drop frames to achieve your desired speed.

To slow down your video, you can type:

ffmpeg -i input.avi -vf "setpts=2.0*PTS" output.avi