Just to be totally clear, Flash Player is a popular target for bad guys to impersonate. In this instance, they're offering you a fake Flash Player. As long as you don't download and run it, you should be fine.
That said, there's a reason you're seeing malware being offered up. If you're seeing this a lot across a bunch of websites (particularly reputable ones), you might want to power-cycle your router and once it comes back up, update the firmware. A lot of the commodity home routers are vulnerable to a number of worms, but they only live in memory, so rebooting and then updating the thing will probably solve it if that's what's going on. You might also want to run a good anti-malware/anti-virus utility on your computer, just to build some confidence that your machine isn't already infected with something else.
At that point, you've ruled out the issues on your side, and if the problem is limited to a couple of sites -- particularly if they're off the beaten path -- then, you might want to consider steering clear.
As with *all* software updates, make sure that you're getting them directly from your Operating System's App Store, or directly from the vendor's site. Don't click through on links or popups. It's easy to get tricked, and as software security gets better and better, humans increasingly are becoming the weak link to target.
For Flash Player, you can get an update here:
https://get.adobe.com/flashplayer/
While you're installing, choose Allow Adobe to Install Updates Automatically, to ensure that you're always protected with the latest security fixes, and remember to exit your browser and/or restart your computer once in a while to ensure that your running applications get patched consistently.
Hope that helps!