IVideoStream and IAudioStream allow to change media speed:

IMediaInfo inputFile = await FFmpeg.GetMediaInfo(Resources.MkvWithAudio);
string outputPath = Path.ChangeExtension(Path.GetTempFileName(), ".mp4");

IVideoStream videoStream = inputFile.VideoStreams.First().SetCodec(VideoCodec.H264).ChangeSpeed(1.5);

IConversionResult conversionResult = await FFmpeg.Conversions.New()
    .AddStream(videoStream)
    .SetOutput(outputPath)
    .Start();

ChangeSpeed() method accepting 1 argument - multiplayer. Multiplayer has to be between 0.5 and 2.0. If you want to speed up streams, use values greater than 1, if not, less than 1.