With specific tools it's possible to update to any firmware. I've read on here numerous times you can't update to 9.2 (for example) because there is no cartridges with that update and the online updater will give you (at the time of writing) the 9.5 update. There is however one other method of updating to a desired firmware version.
Upon joining a local multiplayer / download play session you may be prompted to update your system. The host 3DS acts as a server and delivers the firmware it is running.
For example, I have an emunand 9.2 on system A. System A creates a local mario kart 7 lobby from launching the game via a mk7 .cia installed in emunand. System B joins the lobby and is prompted to update. System A will now send core files for the 9.2 update to system B without either requiring a valid internet access point.
Note this does not occur if loading a game from a .3ds rom or retail cartridge (unless that firmware version is required to play that particular game)
Upon joining a local multiplayer / download play session you may be prompted to update your system. The host 3DS acts as a server and delivers the firmware it is running.
For example, I have an emunand 9.2 on system A. System A creates a local mario kart 7 lobby from launching the game via a mk7 .cia installed in emunand. System B joins the lobby and is prompted to update. System A will now send core files for the 9.2 update to system B without either requiring a valid internet access point.
Note this does not occur if loading a game from a .3ds rom or retail cartridge (unless that firmware version is required to play that particular game)