Go to Settings and Configurations at the top right of your screen. This will take you to this page:
Within the horizontal navigation bar select Accounting. Then, select Invoice Types. This will take you to here:
To add an invoice type, click the yellow + Add button in the bottom right-hand corner. Your page will now look like this:
Type in the Name and select/deselect the checkmark to the right of the name to make this active/inactive.
When done, click the green Update button in the bottom right-hand corner to save changes.