You must be logged in under the exact same username Admin account that registered the blog in question to change the URL.
Clicking this link should display all your blogs registered under the same username, including the hidden ones: https://dashboard.wordpress.com/wp-admin/index.php?page=my-blogs&show=hidden
If you do not see the blog here http://en.support.wordpress.com/my-blogs/ then
follow this guide > http://en.support.wordpress.com/my-blogs/#my-blogs-dashboard-visibility
If you have lost your log-in information you can visit https://en.wordpress.com/wp-login.php?action=lostpassword
If required click the "Need More Help" link and complete the form for account recovery.
Then you can use this guide http://en.support.wordpress.com/changing-blog-address/
That option will permanently disable the address and you will not be able to reuse it. Only select this option if you are sure you will not use the address again.
If you want to redirect traffic to the new blog, and retain and page rank the blog has or authority your have gained in the niche then you can keep the address and purchase the Site Redirect upgrade.