Add slightly more detailed guidance about upgrading to the docs (#40227)

---------

Signed-off-by: merobi-hub <merobi@gmail.com>
(cherry picked from commit 16b17f7)

docs/apache-airflow/installation/upgrading.rst

@@ -57,6 +57,16 @@ when you choose to upgrade airflow via their UI.
 How to upgrade
 ==============
 
+Reinstall Apache Airflow™, specifying the desired new version.
+
+To upgrade a bootstrapped local instance, you can set the ``AIRFLOW_VERSION`` environment variable to the
+intended version prior to rerunning the installation command. Upgrade incrementally by patch version: e.g.,
+if upgrading from version 2.8.2 to 2.8.4, upgrade first to 2.8.3. For more detailed guidance, see
+:doc:`/start`.
+
+To upgrade a PyPI package, rerun the ``pip install`` command in your environment using the desired version
+as a constraint. For more detailed guidance, see :doc:`/installation/installing-from-pypi`.
+
 In order to manually migrate the database you should run the ``airflow db migrate`` command in your
 environment. It can be run either in your virtual environment or in the containers that give
 you access to Airflow ``CLI`` :doc:`/howto/usage-cli` and the database.