You can find FFMpeg compilation instructions from https://trac.ffmpeg.org/wiki/CompilationGuide
If you can't find a pre-built version of FFMpeg, you'll have to compile it from the source. By using the above link, you'll be getting the latest snapshot and it may not be the version you want. If you need to compile a specific version, follow the instructions below.
Pick the correct version you need from https://git.ffmpeg.org/ffmpeg
On the guide, follow all instructions upto 'FFMpeg' section, then instead of the snapshot build, switch to the version needed. (Using v4.3 on the example below)