It has 3 10mm nuts. one under the plastic triangle cover above the door panel. 2 behind the door panel. These three 10 mm nuts hold the mirror on the door. The door panel should be removed to address 2 of the 3 nuts.
In addition, on some models, typically those with tow mirrors as original equipment an additional brace is attached to the door with 3 addition 10mm bolts that attach to rivnuts in the door. Do not be concerned either way if this brace is not present upon door panel removal, it is a brace but doesn't effect mirror operation. However if you have tow mirrors and the brace is not present then this could be a potential source of mirror shaking. Only way to determine is take door panel off.
I hope this helps
PS. If all are tight. It could be the mirror housing itself which could require mirror replacement or disassembly for repair