You can change most of the texts through the Administration panel.
If you want to change a given text that isn’t on the Administration panel, and belongs on Decidim code, you should first find out which key is being used. For instance, you want to change the home page text where it says "Let’s build a more open, transparent and collaborative society.", you would search the text (using github or grep) and then you would extract that key and their parents. On this case it’d be:
en: pages: home: footer_sub_hero: footer_sub_hero_body: Let's build a more open, transparent and collaborative society.<br /> Join, participate and decide.
You need to create a file for this translation as Ruby on Rails i18n documentation says, for instance config/locales/home.en.yml