How can I rename the "portfolio" slug and its texts to something else?

To rename that, please open "/includes/plugin-codes/custom-uxbarn-portfolio.php" file. You can then find the portfolio texts in the function of "uxbarn_custom_port_cpt_args()".

For the portfolio taxonomy, just see in the function of "uxbarn_custom_port_tax_args()".

If you get 404 page after making changes, please go to "Settings > Permalink" and click to save once. This should solve the 404 issue