How do I change my default font on my mac with version 10.12

A utility called TinkerTool can be used to change the system font but only for certain uses. it cannot change the main menubar font, for example. OS X does not have provisions for general system font changes.

You can not change the System Font.

You can set the readability a bit better in SystemPreferences-Accessibility.

You can find "work-arounds" on the internet on how to change it (for Yosemite, ElCapitan and Sierra), but there are issues with all these work-arounds, I strongly advise not to use them (for all, it creates a wrong system-font-folder).

Lex

You can't change the system font.

The same way you did in all previous versions of OS X.