How to edit / translate theme strings (captions, words, phrases)
The easiest and quickest way to translate theme phrases (not part of content edited in the dashboard) use a free Loco Translate plugin. After its installation go to "Loco Translate > Manage Translations" in WP dashboard. You will see a list of translation packages - usually one for each theme and plugin installed.
Now you need to detect where is your target phrase is stored. Usually intuitively you can have an idea does the phrase may belong to a theme and can be translated in the theme package, or it is belongs to a plugin. You can also use a method of exclusion - search first in a theme translation file for a target phrase, and if its not there - check the plugins translation files.
Here is a beginners guide on how to work with Loco Translate.