Theme Issues)*
If you’re uncomfortable with FTP and the idea of deleting core files,
there is a plugin available for downgrading WordPress. If you like this
idea, go ahead and install WP Downgrade
<wordpress.org/plugins/wp-downgrade/> after backing up your site.
Ref: kinsta.com/blog/downgrade-wordpress/
*Disable Plugins and Change Themes from phpMyAdmin*
In phpMyAdmin, Select the wp_options table, Locate the row called
active_plugins, Delete everything in the option_value column on this
row, click Enter.
In phpMyAdmin, Select the wp_options table, Locate the rows called
template and stylesheet, Modify the option_value for both of these rows
to a new theme name, ex: twentynineteen, click Enter.
Ref:
wpengine.com/support/disable-plugins-theme-phpmyadmin/#Change_Theme_from_phpMyAdmin