vim /etc/chromium-browser/customizations/00-rpi-vars
look for
change it for
if you want to put the last version, you can look the number here https://get.adobe.com/es/flashplayer/
Then is open the chromium and go to the web you want, and click the padlock at the left of the url , click site settings, click flash, allow
If flash is updated, go here https://archive.raspberrypi.org/debian/pool/ui/r/rpi-chromium-mods/
and install the last armhf .deb , and change the version on /etc/chromium-browser/customizations/00-rpi-vars , you can see here the version number https://get.adobe.com/es/flashplayer/