docs: fix manual installation upgrading instruction

Author: kenjis

user_guide_src/source/installation/installing_manual.rst

@@ -32,10 +32,9 @@ None
 Upgrading
 ---------
 
-Download a new copy of the framework, and then follow the :doc:`upgrade instructions <upgrading>`
-in the release notice or changelog to merge that with your project.
+Download a new copy of the framework, and then replace the ``system`` folder.
 
-Typically, you replace the ``system`` folder, and check designated
+Read the :doc:`upgrade instructions <upgrading>`, and check designated
 ``app/Config`` folders for affected changes.
 
 Pros