While it is technically possible to update your J3 installation to J4, it can be a tricky process because some extensions you may have installed over the years may not be compatible with Joomla 4. If you're going to attempt it, I would recommend that you use Akeeba backup to first make a copy of your site as a Staging server. Once you've done that you can experiment with updating your site on another machine that doesn't affect your production installation.

Important: The older Leohtian template is NOT compatible with Joomla 4. Unfortunately this means that a Leohtian installation will not be a simple update.

You will need a new template, one that's confirmed to work with J4. The default J4 Cassiopeia template is excellent, you can customise that if you feel comfortable doing so. You can create a child template of Cassiopeia and work with that to ensure that you can update Joomla in the future without fear of your template customisations being lost.

Jomres is cross-platform compatible. This means that providing your installation of Jomres and it's plugins are up-to-date, then you can update Joomla.

Once you have setup your Staging server, you should do things in this order :

• Make sure that Jomres and its plugins are up-to-date. In Site Configuration change the template version of Jomres to Bootstrap 5.
• Update any other plugins that you can update.
• Start the Joomla 4 update process. Take care to uninstall any plugins that cannot be confirmed as compatible with Joomla 4.

Once the update has been completed and you've worked through any issues you will need to re-configure your site to work with your new template.