url change? domain change? confused

Hello,
I am very confused of what I have to do.
I have installed wordpress in to a subdirectory
like this:

http://www.mydomain.com/wordpress

it is working fine, etc. My problem is I want now, after testing my website, users to go to http://www.mydomain.com without any subfolders and go to the first page of the wordpress installation.
how can I do that?
also, there was one time a while ago where I installed a wordpress in a subfolder, I pointed the domain to that subfolder so the domain will go into the index.php by default and nothing will show up because the url was not the same in the wordpress settings. I am afraid to mess up my new installation. if I do, where can I change the url in the database? becuase if I mess up the url in the settings I won’t be able to access the control panel anymore.

Thank you