HTML5 allows your browser to present Rich Media Content natively (making browser plug-ins like Flash/Atom/Silverlight obsolete). You shouldn't ever need to download a "replacement". if you have desktop apps that require Flash (this is rare) they are most likely utilizing Adobe Air and should continue to function. The shift to HTML5 is only something for content publishers to be concerned with; you've basically been using HTML5 for over 2 years and probably haven't even noticed.
(btw, Microsoft just recently began offering a Knowledge Base update that will remove Adobe Flash from your system entirely, in anticipation of its end-of-year deprecation. I ran the patch over a week ago on both a Win 8.1 Media Center Edition system and a Win 10 Pro system and haven't run into a single issue.)