Having issues changing URL-self hosted

Hi guys

Hoping you can help with my query

I added a self hosted WP blog to my domain rachael-knight.co.uk as rachael-knight.co.uk/WordPress/wordpress whilst I was getting to know the system. I now want to change the WP URL to ideally trigeminal-neuralgia.rachael-knight.co.uk, or alternatively rachael-knight.co.uk/trigeminal-neuralgia

The WP URL and site URL are both grayed out in my general settings

I’ve tried to update the folder structure in the FTP and changing the wp-config file but nothing is working. In the wp-config do I change both WP_HOME and WP_SITEURL to the new one, or just one? I swear I’ve tried all possible ways. I wouldn’t even mind leaving the files where they are, and setting up a redirect.

I’ve already tried following these
https://codex.www.ads-software.com/Changing_The_Site_URL
https://codex.www.ads-software.com/Moving_WordPress
which aren’t helping, so I mustn’t be understanding something

Hope you can help – much appreciated
Rach