How can I fix the plastic "chin" under the display?

First it’s glass not plastic! Be very careful as just behind it is the ribbon cables which drive the display and camera system. It doesn’t take much to damage them and once damaged it’s time for a new display!

Ok now that I’ve scared you, using a thin knife careful lift off the loose broken pieces then using a hair dryer being careful to only heat up the plate (use some foil to protect the keyboard and screen) again use the knife to just grab the edge of the smaller segments. I use some good sticky tape to make pull to help lift off the bigger pieces.

Sadly Apple hasn’t published the newer M2 model repair manual yet. Here’s the M1 version which I’m suspecting uses the same basic parts M1 13” MacBook Pro sadly the chin plate is not available from them but I think you can still fine them online as I’ve gotten them before.