San Francisco is a new font you'll be seeing a lot more of in the coming months, thanks to its debut on the Apple Watch. Now the font is available for download and installation on your Yosemite-based Mac, thanks to a new project available on Wells Riley's GitHub page.
These San Francisco fonts have a special name table with names identical to those of the system fonts. Because the font folder /Library/Fonts takes precedence over the fonts which are in /System/Library/Fonts, these specially crafted fonts are used for the user interface instead of the real system fonts. The original system fonts are not deleted or modified in any way.
This only works on Yosemite-based Macs and it will only replace Helvetica Neue with San Francisco, which is another sans serif font that's even more lightweight than Helvetica Neue is.
You can find the files you need and instructions (including steps on how to later undo this, if you wish) on the Github page. It requires some skill, a deep obsession with fonts, and there are several disclaimers. So, read carefully and proceed at your own risk.
Source: Wells Riley