You can change the portfolio item slug from Theme Options > Portfolio > General > Single Item Slug:

Theme options > Portfolio > General > Single Item Slug
After change please go to “Settings > Permalinks” and click “Save changes” button.
You can change the portfolio item slug from Theme Options > Portfolio > General > Single Item Slug:

Theme options > Portfolio > General > Single Item Slug
After change please go to “Settings > Permalinks” and click “Save changes” button.