log into your wordpress admin: domain.com/wp-admin
go to settings
change wordpress address url and site url to the new destination.
access your FTP account and move all the files into the new directory
That seems like the most logical approach.
If you were to move the files before making the change in wp-admin, it will fail. Your wordpress is configured to work in a specific directory. If that files are moved into a new directory, your wordpress will no longer work, including the wp-admin page. Therefore, you will need to make the changes first in wp-admin and then reflect the changes by transferring all your Wordpress files into the correct directory.