gistfile1.py

import uuid
fieldname = 'UUID' # set the name of your field to update

layer = qgis.utils.iface.activeLayer() # get the active layer from the TOC
layer.startEditing() # start editing

iter = layer.getFeatures() # get all features from the layer
for feat in iter:  # iterate over features
    feat[fieldname] = str(uuid.uuid4()) #fill field with GUID
    layer.updateFeature(feat)

layer.commitChanges() # save edits to the layer