How to change strings in the theme/core?

When I use Joomla I can change a core string using the language files. So where it would say "Cart" you would go into the language file and change it to what you want it to say.

I want to be able to do this for one of our client's websites which uses WordPress. Is this possible? I am using the wpex-adapt theme.

The exact bit I want to change is on the Portfolio page on the individual item page where it says "newer" and "older" which I want to change to "previous" and "next".

Any help will be greatly appreciated, thanks.