Can't access wp-admin after changing url to the website

Good morning everybody,
This morning I’ve tried to change the url to my wordpress website from gregevain.com/wordpress into https://www.gregevain.com.
I probably made a mistake by doing it directly from the settings panel in wp-admin.

I can’t access either the blog (i have still something with gregevain.com/wordpress but the css seems not to be taken into consideration anymore) or wp-admin anymore (neither with gregevain.com/wordpress/wp-admin nor with https://www.gregevain.com/wp-admin, etc).

Could you please provide me with a solution to access wp-admin and reset the url ?

Thank you very much.

Regards,
Charles