1. Portfolio Slug
In WordPress Dashboard go to Appearance submenu and click on Customize.
After clicking on customize you will in theme Customizer.
Here you have to click on Custom Slug than Post Slug
Here you will manage Portfolio Slug.
Go to Customize -> Custom Slug -> Post Slug -> Portfolio Slug
2. Portfolio Category Slug
In WordPress Dashboard go to Appearance submenu and click on Customize.
After clicking on customize you will in theme Customizer.
Here you have to click on Custom Slug than Texonomy Slug
Here you will manage Portfolio Category Slug.
Go to Customize -> Custom Slug -> Texonomy Slug -> Portfolio Category Slug