With Pix4D open go to Help -> Settings -> Camera Database tab -> Export
Save the camera database xml file somewhere on your computer.
Open the xml file in a text editor like Notepad (should be able to just double-click to open, otherwise open your text editor and drag the file into the open editor window, File -> Open)
Download the camera profile that corresponds to your modified camera.
Open the xml camera profile in your text editor
Copy the text from the <camera> to the </camera>
On the camera database file that is open in your editor, paste the copied text after one of the other </camera> lines. If no </camera> lines exist, copy it after the <vendorId value="pix4d"/> line and before the </cameraModelDB> line.
Save the camera database xml file
Go to Help -> Settings -> Camera Database tab -> Import
Select the modified camera database file
Start a new project or open an existing one.
Click the Camera icon (Image Properties Editor) at the top left of the main window. If this is a new project, go to the next step.
Click the "View..." button under the "Selected Camera Model" section.
Click the drop-down from the "Camera Model Name" and select the profile you have added, which should have a "user" icon (upper half of a person profile icon).
Select the "OK" button at the bottom and run your project.