On Android :

The saves are deleted if you uninstall the game but they shouldn't be deleted when you update. The save location is "Internal Storage" -> "Android" -> "data" -> "kk2oven..."

On PC :

The saves are in "C:" -> "Users" -> "your username" -> "AppData" (hidden folder) -> "LocalLow" -> "KK2Oven"

On IOS :

No idea

On Newgrounds :

I don't know, it depends on what browser you use I guess. But as a general rule, you should play only offline versions of the games as they are better quality and more stable.