Cant upgrade to new client

Hey :) i had the new Client before but dont liked it. I deinstalled league and downloaded it again to have the old Client permanently. Since the sandbox mode just came out i wanted to upgrade league again but when i clicked the upgrade button nothing happend and when i try to click the start button a message Pops out saying "league Client error. Please install league Alpha and try again" i cant install the new Client again and i tried deinstalling lol and installing it again and upgrade the Client again. it didnt work:/ Need some help. Thank you :)