On your device, go to Settings.
Then, go to Apps and click Manage Apps.
Find the Lumin app.
Open Lumin, and then click Uninstall.
Then, tick this box if you'd also like to delete all of the data associated with Lumin from your Google Chrome profile.
Lastly, click Uninstall.
How to uninstall the Lumin desktop app (Pixelbook)
A step-by-step guide on uninstalling the Lumin app from your Pixelbook.