The theme and your data are kept separately. Updating the theme does not affect the data. So you can just download the latest theme version from ThemeForest, go to your admin panel and deactivate the current theme, delete it, and install the new one.
The theme is translatable (comes with a pot file). To translate you can use a free plugin called Loco Translate.
Once you install it, you would need to create a translation by clicking ‘New language’ (even if you see your language on the list):
Then you can find the phrases you want to translate and translate them (don’t forget to Save):
It’s handy to use browser search here as Loco Translate does not provide a search field.
And finally, go to Admin Panel > Settings > General, and change the language to the one you want to use. This will activate the translation you’ve just created.