Text areas in the theme can be edited and changed either through the Theme Editor or within the Language files, depending on what kind of text it is!
Text in homepage sections, product.details and page.details content blocks can all be changed within the Theme Editor in their section/content block settings. There are a few other instances where in the theme can get changed within the Theme Editor, so it's always good to check there first when wanting to update something!
If the text you're looking to change is not editable in the Theme Editor, you'll want to head over to the theme's Language files (Online store > Themes > Actions > Edit Languages).
The Languages files allow merchants to directly change the labels, notes, or other text areas found throughout the theme. Here, fields are organized under which areas they belong to, however, you can quickly search the files for the text you'd like to change as well.
For example, if you want to change the 'Add to cart' button text, you could open the Products tab within the Language files, then locate the 'Add to cart' portion to modify the text – or you can simply search Add to cart to pull it up quickly:
More articles on the language editor: