Sarah,
You choose the URL during the Landing Page setup phase. The "Name" of the page you see in Marketing Activities or Design Studio is just your internal name and IS NEVER SEEN by leads.
The URL slug is what is seen. You can see this URL slug by clicking on the landing page and looking at the summary.
If you do need to change a page's URL slug, you can do what Erik says.
Landing Page Options > URL Tools
Then you can edit it.
Option 1: Keep Both URLs creates a 301 redirect and you can use either URL. This is good if you accidently sent out the bad URL or if you realized it is not SEO friendly.
Option 2: delete old URL. Only do this if your bad URL was never seen by anyone outside the firm or linked to from outside.
Marketo URL formats are usually:
yoursubdomain.yourcompany.com/page-name-url.html
I'm using dashes there because you should for SEO purposes.