Hi there,
Do you want to remove all the content from your site (including posts and posts), then reset changes from the theme? If so, something like that is usually ideal: https://ww.wp.xz.cn/plugins/wordpress-reset/
@filipe I do not want to delete my content. I only want to delete TT3 completely and start again. I am testing and learning, and making huge mistakes for instance in header and footer.
I believe that part of the TT3 setup is hidden in the “big american file”, the ugliest part of WP – also named: wp-options, a table in the database. That is why it is not enough to delete the theme folder using you FTP.
Oh, I see! Thank you for clarifying that.
Your theme changes are in the database (not in a specific file).
Generally, for going more specific on the theme, it really depends on what you want to revert. There is not a single button to reset all the theme options.
If you would like to revert the style changes, you can go to the Site Editor > Styles, then the revision icon and you can click on “Reset to defaults”.

Full Size: https://d.pr/i/sAQNV1
If you would like to reset changes for a header/footer or native pattern, you can go to the Site Editor > Patterns, then select what you want to revert and click on the three-dot icon (kebab menu). There, you can cleat customizations.

Full Size: https://d.pr/i/b9U6ZW
PS: For future reference, you can also use something like https://playground.wordpress.net/ for testing and learning before making changes to your site.
I am working in a clone of the original webpage.
My problem is that I want to move the final result to the original webpage. Before I could do it by establishing a child-theme. Now I do not know how.
Second, I believe that by cleanup my database – especially remove unused rubbish from the wp-options table – will make life easier and faster.
Hey @boldt! Is your clone in a template or in page? If it’s in a page, then you can try setting up a fresh testing WP installation, and copy-paste that page’s content there, to test.
In general, if you’re looking to ave your work as a child theme, you should check the Create Block theme plugin, which will let you export your changes to a child theme.
I have established a complete copy of the original WP somewhere else. I am not using any plugin – I have done it by old fashion hard work upload/download – phpMyAdmin – copy/paste – search/replace…
I want to give the page a complete overhaul: upgrade from php 7.4 to 8.1 (or higher) replace old plugins with new, remove “artificial respiration” of old plugins….
I will take a look at the sugested plugin.