1. should I uninstall the actual theme or just overwrite the sketch folder under themes folder by moving the new unziped folder?
Either way – if you want to be safe, you can remove the old folder and upload the new one. If you had made any other changes to the theme files, be sure to save a backup of the theme folder with your changes so you’ll be able to transfer those tweaks into your new child theme.
2. should I export an xml to keep the settings, pages & things like that before I do it and import after the fresh install or this process keep things like they are now? (I believe the answer is no, but is better ask than make mistakes…)
Your content and settings should remain intact as they’re not part of the theme files – so no need to do an export.
3. the little tweaks I made on CSS were made via Custom CSS, so I don’t need to worry about it, right?
Right! If you used a custom CSS plugin to make CSS changes, they’ll be preserved when you upload a fresh copy of the theme — and whenever you update the theme in the future.