Python Dictionary contains List as Value - How to update?

An accessed dictionary value (a list in this case) is the original value, separate from the dictionary which is used to access it. You would increment the values in the list the same way whether it's in a dictionary or not:

l = dictionary.get('C1')
for i in range(len(l)):
    l[i] += 10

>>> dictionary = {'C1' : [10,20,30],'C2' : [20,30,40]}
>>> dictionary['C1'] = [x+1 for x in dictionary['C1']]
>>> dictionary
{'C2': [20, 30, 40], 'C1': [11, 21, 31]}