Only on Windows:
0. DO A NAND BACKUP
1. "Acquire" the 3DS CTR SDK. Google is your friend here.
2. Use the CTR SDK to convert a Windows font to to a bcfnt font. It's in a some 'tool' folder. You can preview how the font will be if you export it as an image. Play with the values of the program and test diferents results.
3. Then use astronautlevel's Font Tool to convert the .bcfnt to a CIA
https://github.com/astronautlevel2/FontTool. See their github page for instructions.
4. Then you have to NCCH Encrypt that CIA with Decrypt9.
5. Install that CIA.
6. Reboot console.
If anything gone wrong, restore backup and ask here.