1. backup nand using gateway menu, incase u screwed (aka bricked)
2. create emunand using gateway menu, the emunand should be the same as your sysnand ver, in your case 9.0
3. download all 9.2 fw using 3dsnus, u will get around 100+ cia (make sure u select new 3ds)
4. go to emunand, launch fbi and install all cia. u should received an error saying file already exist, just ignore and proceed until finished.
5. reboot to emunand, u should be successfully install 9.2 into emunand. make sure u're able to boot successfully
6. use emunandtool (window application), extract emunand into pc.
7. rename the emunand to NAND.bin, put NAND.bin into SD card
8. go to gateway menu, highlight downgrade option, hold UP key and press A to launch restore.
9. restore and after your restart, your sysnand should be 9.2