You will need to download an older firmware and install it with Odin. However, if it is just a theme and some apps I would just fix those. Try another launcher, I use Nova Launcher. https://forums.androidcentral.com/e...com.teslacoilsw.launcher&hl=en&token=qtM_S35V Downgrading is a big process just for some small fixes you are after.
Chrome and Youtube won't be fixed by an older OS. It is the actual app playing up, how are they not efficient?