If you want to change the actual face (eyes, nose, mouth, chin, etc) you use setFace command
myUnit setface "face15"
Thats's general Guba's face.
If you want to change the expression (smiling, angry, confused, etc) you use setMimic command
myUnit setMimic "smile"
Should make the unit smile, but I'm nor sure how long the smile lasts since I never actually used the mimic command.