I updated my firmware to 10.2 but I forgot to install a 2.5 ironhax save, does that mean I can use SmashHax? I have a New 3DS.
Yes. 10.2 N3DS can use SmashHax 2.5, then install Ironhax 2.5. However, at the moment it only works in Linux based OS. Use this guide
https://gbatemp.net/threads/tutorial-using-smashhax-with-linux.397194/
You can download Ubuntu and burn to a DVD-RW disc (not sure CD-RW will work or not) or make an Ubuntu boot USB. They work directly on the DVD/USB, and doesn't install anything on your PC, so after done with SmashHax you can wipe the DVD/USB to reuse for something else.
You can use Virtual machine too, but you have to use a USB Wifi adapter that can be be detected with Linux based OS as USB device (built in WIFI card does not work).