You can change it in the New Document Profiles. You may search for its location, depending on OS. It may be something like this in an earlier thread:
Thanks Jacob and Monika, Having wandered through the threads I found some helpful but none could get the final result unfortunatly, maybe because I am using Os10.8.5. The best was on the Finder, hold alt select GO > select Library from the drop down menu > Adobe CS6 > New Document Profiles > I chose Basic RGB.ai select Window > Type > Character Styles. Normal Character Style is highlighted in blue, double click, select Basic Character Formats and then make the relevant changes to Helvetica 8pt. Click OK, close Character Styles window, to Save I had to type something, Saved, then quit. Restart CS6 open ne wwindow, and Hey Presto Myriad Pro 12pt is still the default type!
I'm not on a mac, I'm using Windows 7, but there are two places where copies of these files are.
One is in the user folder:
C:\Users\me\AppData\Roaming\Adobe\Adobe Illustrator CS5 Settings\en_US\New Document Profiles
and the other is in the program files in the system
C:\Program Files (x86)\Adobe\Adobe Illustrator CS5\Support Files\Required\New Document Profiles\en_US
When opening a new document Illustrator will look for the document profiles in the user folder. The Browse button will go to the location last used which after installation or resetting the settings will be the document profiles in the user folder.
The files in the system will be used for creating the files in the user folder when deleting/resetting the settings.
I'm writing this in case it was the system files that were modified. It is also possible if there are more users of the same computer, someone to have made a copy of the documents profiles and the Browse button takes there.