After downloading the service manual
@stooge supplied, the info you're looking for is on page 1183. Search "instrument panel trim plate".
View attachment 221864
In order to get to that point, you'll have to remove a few other trim pieces first though. I had to remove the same trim pieces covering the screen surround when removing my cluster. They all just pop off with a bit of force. I documented the process on the Buick Forum in this thread:
How To: Instrument Cluster Removal
Note: I have the 8" display, but it's the same procedure. You'll only have to go through Steps 1-3, and possibly Step 4, before the surround plate is accessible. I cannot stress enough the importance of using plastic trim removal tools!!! They can be purchased at just about any auto parts store and are invaluable for any trim work.