Make sure you are in the directory for your WordPress site, then go to the wp-content folder. Open the themes folder. Find the folder theme you wish to remove, then right-click on it and select delete.

Likewise, you should clean up plugins, periodically. As plugins represent a greater number of elements you add more frequently to the website, the cleaning process should occur more frequently than for themes.
You should keep in mind that, before switching to another theme, you must make a backup of your current version of the website. Thus, if something goes wrong with the theme change, you can always revert to the previous version, without any risks.
In case of a Child Theme, there’s no Delete button for the Parent theme. This way, you’ll be sure that if something is broken, you can refer back to the Parent Theme.

Why uninstall themes not used

1. The main reason why you should delete unused WordPress themes installed on your blog is that they can slow down your site. Themes take disk space and increase the size of your backup file.
2. The themes might lead to security breaches
3. Those themes might clutter the Admin Dashboard of your WordPress
4. Unused themes removal ensures improved performance for the website.

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WordPress Maintenance includes updating theme, plugins, and core framework to the latest version of WordPress. All updates include external backups to cloud servers.

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WordPress powers nearly a third of the entire internet.

Yet despite the fact that WordPress gives everyone a chance to build a competitive website, WP sites can be notoriously slow.

Because page loading speed can impact your visitor bounce rates, it’s essential to ensure your site loads as quickly as possible.

De-activate Akeeba admin

De-active Secret Key

No Access to Secret Key

If you DO NOT have access to the Administrator / Administration panel of the website because you have enabled 2FA and you know can't log in, you'll need to disable it via PHPMyAdmin 

1. From your hosting account log into PHPMyAdmin
2. Find the table ending in  '_extensions' (the first few digits/letters vary by installation)
3. Find the plugin named plg_twofactorauth_totp and change its 'enabled' status from '1' to '0'
4. Save

This disables the 2FA plugin and thus gets rid of the login with the secret key.

WordPress is now used on 30 percent of the top 10 million sites. When you consider there are an estimated 2 billion sites, getting to the top of the heap is a noteworthy accomplishment.