How To Remove the Stock Air Vents
Only the rotating flaps can be removed unless you also remove your entire dash in the process.
Use a flat head screwdriver, cover the end with masking tape (to prevent scratches) and pry the flaps out - you will notice the area is slightly flexible and the flaps should pop out after a bit of work (see pictures below). The rotating bracket can't be removed unless you take the entire dashboard out.
NOTES: If you are replacing an air vent with a race gauge, the correct size is 2 5/8.