Moving WordPress from a subfolder to a root folder can be very quick and easy. This example will explain how to move it from a subfolder like example.com/wordpress to the root folder example.com.
For simplicity, this tutorial will use the subfolder 'wordpress' for instructional purposes.
The first thing to do is copy the files from the wordpress subfolder to the root folder. This can be done with ftp or Linux command line.
Once the files are moved the root folder, logging into the new admin at example.com/wp-admin can be problematic. So, here is what you can do after you moved the files.
Login to your Cpanel or phpMyAdmin. Hopefully, you have phpMyAdmin or some other mySQL admin manager. Once you have logged in, open up your database. At this point, you will have access to all the tables.
A quick way to find the old url in database tables is to search all tables for the string like 'example.com/wordpress'. You will then see a list of tables like wp_options. If you used a different prefix than the default, the table would be yourprefix_options. In this table, you will want to change the option_name fields which are site_url and home. They are the first two items in the list. Meanwhile, you may want to change recently_edited.
Now, if you have old links you may want to update them. The code below will remove the wordpress subfolder from the url so that image links and other links will work within the current folder.
SET `post_content` = replace(post_content, 'example.com/wordpress', 'example.com')
SET `guid` = replace(guid, 'example.com/wordpress', 'example.com')
Asides from that, you may want to check your .htaccess file and make sure rewritebase is 'RewriteBase /' and not 'RewriteBase /wordpress'.