If you’re not a big fan of the revamped version of Helvetica Neue that ships with OS X Yosemite, I have great news for you! You can replace the system font fairly easily in OS X by downloading Input by Font Bureau. Credit for this little trick goes to Jeffrey Zeldman, who posted about it on Facebook.
First things first, you need to download the Input font to your computer. Just visit the Font Bureau website, scroll down, read & accept the license terms, and then click the big green download button.
Installing the Input Font
Once you have the font downloaded, you then need to install it into your Library/Fonts folder. This can be found by navigating to your Macintosh HD folder and then selecting Library/Fonts. This is NOT the User/Library/Fonts folder.
Saving the Changes
Once the fonts are installed, all you have to do is log out and then log back into your Mac. Then your system font should be changed from Helvetica Neue to Input.
If you’re not satisfied with your new system font, you can always remove it by deleting all of the font files from Library/Fonts and the logging out and back in to your Mac. That’s it!
Even though this method for changing the system font works, it’s not 100% foolproof. There are some apps that specifically call for Helvetica Neue to be used in their user interface elements, and therefore you will not see Input as the font within those applications. If this seems like something that might bother you, I’d suggest not changing your default font, as there is no way around it.