I got "Page Not Found" after changing pages on my portfolio. How to fix that?

Please make sure that the portfolio slug is not the same as any page slug.

For example, the theme by default uses "portfolio" as a portfolio slug. If you also have a page that has the same slug as "portfolio", please change the page slug to something else (eg. "my-portfolio").

OR, you might want to change the portfolio slug instead. To do that, go to "Appearance > Customize > Portfolio Options" and scroll down to the bottom. You will see the options to change portfolio slugs there.

After that, go to "Settings > Permalinks" and click save.

This should help.