user_guide_src/source/installation/upgrade_319.rst
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Upgrading from 3.1.8 to 3.1.9
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Before performing an update you should take your site offline by
replacing the index.php file with a static one.
Step 1: Update your CodeIgniter files
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Replace all files and directories in your *system/* directory.
.. note:: If you have any custom developed files in these directories,
please make copies of them first.
Step 2: Replace config/mimes.php
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This config file has received some updates. Please copy it to
*application/config/mimes.php*.