I heard there is a way to downgrade apps many apps are full of advertising now after I upgrade comes now also in small browser window half year ago I never got unwanted advertising !

That's how the programmer make some money out of a free app. Either it is free, but with limited functions. Or they offer a free upgrade with more options but adding advertising.

I think it is not possible to downgrade.

If you have a copy of it saved on your computer or downloaded you can remove the app and install it via iTunes and use it. They don't stop being signed but the App Store will keep reminding you to update it.