Change Orders can only be added to Fixed Price jobs
To add a Change Order, first go to the relevant project's Contract & Change Orders tab:
At the bottom of your list of line items, there will be a button that reads Add Change Order:
Clicking this button will prompt you for a Description and Price, once these are filled out, you can click Make Active to be able to invoice this Change Order:
To add multiple lines to one change order, click Add Change Order again, and make sure the CO# is the same on both lines.