If you want to change the default slug “ux-portfolio” in a single portfolio URL, please go to “Art Theme(or SEA Theme) > ux-portfolio” to change the slug name:
This article fits the ART, SEA, Arnold, Air theme.
Please make sure the slug name is not occupied by a Page, Category, or Tag. If you set the slug name as “portfolio”, please make sure that there isn’t a Page, Category, or Tag named as “portfolio”.
The slug cannot be empty.
Please notice that it will not affect the “Plain” option of permalinks(Settings > Permalinks)