Most update CIAs found on community sites are region-free, but it is best to match the update region with your base game region (e.g., USA update for a USA game) to avoid compatibility issues.
"Pokemon X Update 15 CIA" almost certainly refers to Version 1.5 (v30720) . Install it via FBI on a CFW 3DS, matching your game’s region, to get the final, most stable version of Pokémon X .
: Fixed an issue causing disconnects during Random Matchups and Online Competitions .