For now, never update past 9.2 or 10.3 system version
Hax doesn't work past 9.6 or 9.9 (i don't remember, but stay in 9.2, it's better, no joke)
9.2 is used to run all the hacks, 10.3 is the last systeme version that can be downgraded.
Keep 9.2 if it's already installed, if not, you will have to update to 10.3 (
DO NOT USE UPDATE IN PARAMETERS since it update until 10.5 today) and then downgrade to 9.2.
1st, let us know which version is used by the 2DS you want to buy
Edit : if you know what a CIA are, you might have already seen some tutorials or something