To Add or Modify your Income Categories, navigate to the Settings>Accounting page.
At the top of the page is the Income Categories section:
To expand this list, click on the arrow to the left of the heading:
This will allow you to modify the names of the current categories on the list. You can also click on the trash can icon to delete them:
The number to the right of Categories tells you how many categories you have entered.
To Add a Category click on the Add Category button at the top right of the page (you can do this with the list compressed or expanded):
A modal appears asking for Category Name. Once entered, click Add Category.
You can add, edit or delete your categories which will allow you to categorize all of your income any way you choose. More information about the Income page can be found here.