Use
3DNUS to download the files, then install the them with a CIA installer such as
FBI. Make a backup of your emunand with the
emunand tool first, just in case something goes wrong.
Steps:
In 3DNUS, enter 9.5.0-23 into the title ID field, and USA, JAP, or EUR into version field, depending on your N3DS region. Select "new 3ds" radio button. Leave "change version numbers to" unticked. Make sure "pack as cia" is ticked.
Once the files are downloaded, a folder will appear in the 3DNUS directory called 9.5.0-23. Copy this onto your N3DS micro sd.
Download the .3ds version of FBI to your gateway micro sd, and run it from emunand. Navigate to the 9.5.0-23 directory and select option to install all cias in that directory. Once it is finished, close FBI and turn off N3DS. Turn back on and go back to emunand. You will now have 9.5.0-23 firmware in emunand (you can confirm this in system settings but remember it will go back to sysnand when you leave system settings).
Have you installed emunand yet? Follow the
gateway manual to setup emunand, then download the .3ds version of
FBI to your gateway micro sd, and use it to install the CIA version from your 3ds sd onto emunand.