I'm doing this guide because a lot of peoples asked how to do that recently.
I won't talk about the n3ds because ninjhax (which is required for the pasta cfw) doesn't work on 8.1J, and there's is no point in upgrading a 9.0 n3ds.
1. Download and launch
3DNUS
2. Type 9.2.0-20 in title id, and your region in version (EUR, USA or JPN)
3. It will create a 9.2.0-20 folder and download a bunch of cias in it, while it's downloading, download
sysupdater (don't use the 0.4b version or you'll brick your console, 0.4 is safe though) and put the cia version on your sd card, then create a 'updates' folder in your sdcard root.
4. After the download finished, copy all the cias files from the 9.2.0-20 folder to the updates folder
5. Start Pasta, install sysupdater.
6. Launch sysupdater then follow the instructions
PS : If you're on 4.X, you can also use PBT instead of Pasta, but never upgrade while in gateway mode or you'll likely brick your console.
Mirror link for sysupdater :
http://jheberg.net/captcha/sysupdater-03/