doc: describe how to upgrade Modules in INSTALL

Fixes #544

INSTALL.rst

@@ -286,6 +286,31 @@ To learn more about siteconfig and run-command files, see the
 also look at the available :ref:`setup recipes<cookbook>` to get concrete
 deployment examples of these files.
 
+Upgrading
+---------
+
+In case you want to upgrade a pre-existing Modules installation, it is
+suggested to:
+
+1. Backup configuration files stored in *etcdir* (like :file:`siteconfig.tcl`
+   or :file:`initrc`);
+2. Remove previous Modules installation;
+3. Install new Modules version;
+4. Review specific configuration of previous installation and adapt
+   configuration files of new version.
+
+It is advised to look at the different documents describing changes and new
+features appearing between versions. By reading these documents you will be
+able to determine the specific configuration options to apply to satisfy your
+use cases.
+
+* :ref:`changes`
+* :ref:`NEWS`
+* :ref:`MIGRATING`
+
+In case you install Modules with the :instopt:`--enable-versioning` option
+set, you may be able to install a new version of Modules alongside existing
+ones (if they also have been installed with this option set).
 
 Build and installation options
 ------------------------------

NEWS.rst

@@ -100,6 +100,7 @@ Modules 5.5.0 (not yet released)
 * Add PowerShell Core (``pwsh``) shell support. (fix issue #326 with
   contribution from Simon Lukas Märtens)
 * Lib: adapt Modules Tcl extension library to properly load on Tcl 9.
+* Doc: describe how to upgrade Modules in :ref:`INSTALL`. (fix issue #544)
 
 .. warning:: Variant names are now fully checked instead of just verifying
    their first character. Only characters within the ``A-Za-z0-9_-`` range are