You currently have land line broadband for your PC upstairs but would like to know if its possible to modify it for wireless for your PC and Laptop which are downstairs?

No, you will have to buy a wireless router, they are nor very expensive and also buy a USB wireless dongle for your computer (if it's not already wireless) and depending on the age of your laptop; if it's modern it will more than likely have wireless built in, if not you will have to buy a wireless card OR a USB dongle.