You can always use emunand to do a 'fake update'. Problem with that is, if you start from a cold boot or exit out of system settings (and maybe other triggers) the 3ds will run off real nand and be in danger of updating again. That's always a big issue with young people so its a bad risk.
Before you do anything else (aka right now), make sure you make a nand backup just in case just such an accidental update occurs. (it will require a hard mod to restore 4.5, but we'll cross that bridge if we get to it - hopefully never)