Try looking at the permalinks settings as well as the portfolio settings. The permalinks settings allow you to create a custom URL-scheme that you can use for many post types.
Although I don’t know the exact theme you’re using, many have options to customize the URL of the portfolio in a portfolio settings page.
Finally, if none of those work, you can look at the theme’s files (assuming you have some experience editing PHP) and find where the custom post type is being created. Depending on how the theme creator created the custom post type, it may be as simple as changing a line or two in the theme editor.
Best of luck to you!