Where exactly to update the composer.json to install an older version of Craft 3?
Make sure you have Composer installed from the command line.
Open your project's composer.json file.
Find the &quot;require&quot; section. Similar to:

&quot;require&quot;: {
    &quot;craftcms/cms&quot;: &quot;^3.6.4&quot;,
    &quot;vlucas/phpdotenv&quot;: &quot;^3.4.0&quot;
},


Edit the craftcms/cms line to the exact version you want. For example 3.5.0.

&quot;craftcms/cms&quot;: &quot;3.5.0&quot;,


Run composer update from the CLI from your project's root and that will pull in a fresh set to Craft 3.5.0 files.