TheoTown

Is there any way I can change modes in game? For example, normal difficulties mode to sandbox mode

You can only change a city into a sandbox one. In order to do so tap (while having the city open) on the bottom bar where the city name is placed. After that you see infos about your city in general. When you tap on the gamemode you can transform it into sandbox.

I see... So it's only available when in the normal modes, isn't it? Thanks by the way

Yes, as as sandbox city would probably have big issues when converted to non-sandbox. Technically it would be possible, of course.