FFmpeg.Conversions.FromSnippet contains few predefined methods to change video format e.g.:

await FFmpeg.Conversions.FromSnippet.ToOgv(inputVideoPath, outputPathOgv);
await FFmpeg.Conversions.FromSnippet.ToTs(inputVideoPath, outputPathTs);
await FFmpeg.Conversions.FromSnippet.ToWebM(inputVideoPath, outputPathWebm);

More conversion types are possible by using new Conversion:

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()
    ?.SetCodec(VideoCodec.H264);
IStream audioStream = info.AudioStreams.FirstOrDefault()
    ?.SetCodec(AudioCodec.Aac);

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