import json
import pprint
import geojson

with open('path/to/test.geojson', 'r') as f:
    data = json.load(f)
    props = data['properties']
    props['prefix:prop'] = 'val'
with open ('path/to/test.geojson', 'w') as a:
    json.dump(data, a, indent=4, sort_keys=True)