BUG: DataFrame.update with tz-aware date doesn't return naive data

doc/source/whatsnew/v0.25.0.rst

@@ -211,6 +211,7 @@ Timezones
 - Bug in :func:`to_datetime` with ``utc=True`` and datetime strings that would apply previously parsed UTC offsets to subsequent arguments (:issue:`24992`)
 - Bug in :func:`Timestamp.tz_localize` and :func:`Timestamp.tz_convert` does not propagate ``freq`` (:issue:`25241`)
 - Bug in :func:`Series.at` where setting :class:`Timestamp` with timezone raises ``TypeError`` (:issue:`25506`)
+- Bug in :func:`DataFrame.update` when updating with timezone aware data would return timezone naive data (:issue:`25807`)
 
 Numeric
 ^^^^^^^

pandas/core/frame.py

@@ -5546,8 +5546,8 @@ def update(self, other, join='left', overwrite=True, filter_func=None,
         other = other.reindex_like(self)
 
         for col in self.columns:
-            this = self[col].values
-            that = other[col].values
+            this = self[col]._values
+            that = other[col]._values
             if filter_func is not None:
                 with np.errstate(all='ignore'):
                     mask = ~filter_func(this) | isna(that)

pandas/tests/frame/test_combine_concat.py

@@ -357,6 +357,13 @@ def test_update_from_non_df(self):
 
         assert_frame_equal(df, expected)
 
+    def test_update_datetime_tz(self):
+        # GH 25807
+        result = DataFrame([pd.Timestamp('2019', tz='UTC')])
+        result.update(result)
+        expected = DataFrame([pd.Timestamp('2019', tz='UTC')])
+        assert_frame_equal(result, expected)
+
     def test_join_str_datetime(self):
         str_dates = ['20120209', '20120222']
         dt_dates = [datetime(2012, 2, 9), datetime(2012, 2, 22)]