Hello firstly I am no Magento expert my background is more design based however over the last year i have got to know my way around the backend of Magento. I already have a store running Mageno birthdaybox.co.uk but would like to update the theme so its responsive, my qestion is can I install the new theme over the old theme keeping all my products or do I have to reinstall Magento again and install all the products over again, thanks to anyone that can help
Yes you can install a new theme on top of your existing Magento, no need to reinstall, if you are buying a theme just follow the instructions parsed along with the purchase. And then you can activate this theme from the administration under:
System > Configuration > Design
You may have some specific functions that are build to work just in your template but otherwise you should be fine.
I hope this helps