It depends.
If you have the original source .fla file and you didn't use ActionScript to build complex interactivity, Adobe Animate CC allows you to publish Flash animations to both Flash Player and HTML5 Canvas. It might be a simple one-click conversion, but it's completely context dependent. You could download a trial and give it a shot.
Alternatively, if you don't have the source, they're just non-interactive animations, or something that would demo well as a video, you could just play them back and use a video capture tool to capture them as video content. The built-in Grab utility on MacOS would get it done. There are plenty of other options that range from free to paid out there.
If neither of those are true -- they're highly interactive and/or use ActionScript, you need to re-work them for the modern web. There's no way to do a simple one-click conversion. Animate CC is probably the place to start if you have the original source files, but it may be a non-trivial combination of saving out individual assets into appropriate web formats and using JavaScript to re-build the interactive logic.