This depends on
when you hacked the 3DS. Hopefully you have Luma on CTRNAND so the 3DS turns on without SD card. Hold (SELECT) while turning on the console. On the top of the upper screen it should say:
Luma3DS v??? configuration
For versions ≤
v7.0.5:
upgrade from A9LH to B9S
For version =
v7.1:
update B9S
For versions ≥
v8.0 : Put latest boot.firm (Luma) from
this 7z archive into the root of your SD.
For the latter two cases repeat the
finalizing setup. Note that the finalizing page assumes you to already have
boot.3dsx in the root of your SD.