How to Create a Joomla Template Override
Choose the output you actually need to override
A template override changes extension or layout output without editing the original extension file. Identify the component view, module, plugin layout, or Joomla layout responsible for the markup. Do not create an override merely to change a color or spacing rule that belongs in CSS.
Open the active site template
In Administrator, go to System → Templates → Site Templates and select the template that should contain the override. Current Joomla help opens Templates: Customise from this screen. Overrides belong to a particular template, so creating one in a template the page does not use will have no effect.
Use the Create Overrides tab
Open Create Overrides and choose the component, module, plugin, or layout item you want to override. Joomla copies the appropriate source output files into the template's override structure. Joomla's documentation notes that this action occurs immediately and reports the location created.
Find the generated files under html
Return to the Editor tab and expand the template's html folder. Joomla documentation shows standard overrides under paths such as /templates/cassiopeia/html/mod_login/ or /templates/cassiopeia/html/com_content/article/. Layout overrides can appear below html/layouts using a mirrored layout path.
Edit only the copied override
Make the required markup or PHP change in the template copy, not in the original component, module, plugin, or Joomla core file. Keep the change as small as practical. A template override is automatically used when Joomla renders that output through the template; it does not require modifying the extension's distributed file.
Test every relevant view state
Test the frontend page as guest and authenticated users where relevant, plus empty, populated, pagination, error, and responsive states affected by the output. PHP errors in an override can break the rendered page, so keep a backup and use Joomla/PHP logs if the result fails.
Review overrides after updates
Joomla's Templates: Customise screen includes Updated Files information for overrides affected by extension or Joomla updates. Review differences rather than blindly replacing the override. The purpose of an override is update-safe separation from core files, but the override itself may still need adaptation when upstream markup or APIs change.
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