By default, you cannot modify Sizzy's built-in devices. However, you can still use these default devices as a base to create custom devices by cloning them and building on the newly created clone. You can also do the same with any custom devices you made.
A common use case for cloning devices is to have multiple versions of the same device with different settings. For example, to test an iPhone with the dark theme and another with the light theme. To clone a device, click the Device Drawer button in the app header to open the list of available devices.
Here, click the ⚙ Gear icon to open the device management window.
Hover over the device that you want to clone, and click the Copy icon to the extreme right.
Now, go to the Custom Devices tab, hover over the newly cloned device, and click the ✏ Pencil icon. This is also where you can modify any custom devices you created from scratch.
Edit the device parameters you want, and save your changes. You will find your custom device at the bottom of the Available Devices section.