Having trouble with FLASK not updating HTML\CSS

I'm having a problem. When I make changes in HTML\CSS and re-run app.py nothing changes on the browser page but instead, it remains the same as it was when I updated it for the first time. I've tried many options from setting configs to hard-refreshing (CTRL-F5, SHIFT-CTRL-R) but nothing helps. So, I think it got stuck on the old (first) update.