What difficulties are you experiencing when attempting to have the URL change? Are you getting any errors, if so what are they?
To have the URL for a space, or any channel changed, you would access the Site Manager through the control panel, and on that space click the action button next to it for the drop down. On that drop down, select "Quick Edit", and you will be able to change the URL identifier for that space.
You can follow up for our Site Manager article on what can be done in there.
If you're still encountering issues getting the URL identifier changed, please send a request to email@example.com
Do note, if you are not an authorized support contact, please have one copied on the ticket when sending in.