Xabe.FFmpeg provide support for changing bitrate on Audio and Video streams:

string inputVideoPath = Path.Combine("C:", "Temp", "input.mkv");
string outputPathMp4 = Path.Combine("C:", "Temp", "result.mp4");

IMediaInfo info = await FFmpeg.GetMediaInfo(inputVideoPath);

IStream videoStream = info.VideoStreams.FirstOrDefault()
    ?.SetBitrate(6000);
IStream audioStream = info.AudioStreams.FirstOrDefault()
    ?.SetBitrate(192000);

return FFmpeg.Conversions.New()
    .AddStream(videoStream, audioStream)
    .SetOutput(outputPathMp4);