From the BIAB main menu, select "Styles, Stylemaker, Edit a Style"; then choose the style file you want to edit. After editing, save it as a different name. You can then use it; however, if you want to see it in the stylepicker, you'll need to rebuild the list and then you should find it in the "Other Styles Found" category.
If you don't know what to do once you get into Stylemaker, I suggest reading the user manual and viewing the "how-to" videos, which can be found at
Stylemaker Part 1 and
Stylemaker Part 2.
Good luck.