Delete user defined brushes

The files themselves or the brushes in each library?

The files are stored at:

C:\Users\Username\AppData\Roaming\Adobe\Adobe Illustrator 25 Settings\en_GB\x64\Brushes

Deleting individual brushes or otherwise changing the libraries requires opening them directly in Illustrator (i.e. File > Open as opposed to 'Open Brush Library') and editing the document brushes (Window > Brushes), then re-saving the file.