OP wants to updateYou downgrade? If you already have a CFW, then there's no benefiting from downgrading.
If you are on stock OFW and on 11.3, there's no way to downgrade the system. Even hardmod/dsiware mod have been patched out due to a NATIVE_FIRM requirement added to the homemenu.
If you are on 11.0, then you can easily hack your system. Just study this guideI buy a new 3DS, as you tell me, But it is in 11.0.0