How to backup old savefiles? I don't know how can someone help?
It depends on your device, in my PC the saves are stored in "(your user)\AppData\Roaming\RenPy\ElectricSheep". The appdata folder is usually hidden by default, so try and google how to "show hidden files/folder" if you can't found it.
How about in smartphone dev I don't even see the renpy and electricsheep in my dev
android > data > electric sheep folder