After you access phpMyAdmin (usually through cPanel or your hosting control panel), browse the wp_options table and then click the Search tab at the top of the screen. Enter in %_GFCache_% for the option_name field and then click Go. You should be returned one or more rows of data (each transient uses two rows. One for the transient data and one for the expiration). Simply delete these transients using the delete link that phpMyAdmin  gives you for each row. You should then find that your form is unlocked and you’ll be able to edit it.

You can also upload a WordPress plugin to delete all transients. Once that is done, you can remove the plugin.
What is Transients? The Transients API in WordPress allows developers to store information in your WordPress database with an expiration time. This helps improve WordPress performance and speed up your website while reducing overall server load.

Transients are supposed to be temporary, so it is usually alright to remove all of them, not just the expired transients.  To be safe, always make a backup before you make any changes to your site.