To be direct: You cannot change your Unvale username, your profile's URL.
However, you can change the display name on your profile's main page.
To change your display name:
Use the "Edit..." dropdown menu and choose "Edit profile".
After you click this, you can see all of the fields you can edit on your profile.
Click the little pencil next to your display name and you type what you want there instead!
If you want to have a different URL you will have to make an account at this time.
Reach out to support to ask for an account deactivation letting them know it's for a username change. If you have any OC pages on your account that you want to save, please transfer the pages to a different account beforehand as they will not be available for transfer once the old account is deleted. Once the old account is taken down it will free up the old username to be claimed.
Unvale's team of developers is hard at work, constantly introducing and improving new features. If you have any questions please contact us!