Pandas Series object does not update values

I got a Pandas Series object looks like following:

(Pdb) data1
date_time
2019-12-31 01:00:00+00:00    1.0
2019-12-31 02:00:00+00:00    1.0
2019-12-31 03:00:00+00:00    1.0
2019-12-31 04:00:00+00:00    1.0
2019-12-31 05:00:00+00:00    1.0
                              ...
2020-11-30 20:00:00+00:00    5.0
2020-11-30 21:00:00+00:00    20.0
2020-11-30 22:00:00+00:00    20.0
2020-11-30 23:00:00+00:00    20.0
2020-12-01 00:00:00+00:00    20.0
Name: ASD, Length: 8064, dtype: float64

values of the object:

(Pdb) data1.values
array([1., 1., 1., ..., 20., 20., 20.])

if I run a function on data1.values called shift()

(Pdb) shift(data1.values, -1)
array([1., 1., 1., ..., 20., 20., 0.])

The function basically shifts the array its given as input. My question is why pandas Series update() function does not work as intended?

(Pdb) data1.update(shift(data1.values, -1))
(Pdb) data1
date_time
2019-12-31 01:00:00+00:00    1.0
2019-12-31 02:00:00+00:00    1.0
2019-12-31 03:00:00+00:00    1.0
2019-12-31 04:00:00+00:00    1.0
2019-12-31 05:00:00+00:00    1.0
                              ...
2020-11-30 20:00:00+00:00    20.0
2020-11-30 21:00:00+00:00    20.0
2020-11-30 22:00:00+00:00    20.0
2020-11-30 23:00:00+00:00    20.0
2020-12-01 00:00:00+00:00    20.0
Name: DEtoDK, Length: 8064, dtype: float64

It should actually add a 0 in the end and shift the series values. Why I cant do that?