hello, I cant install the upgrade as it says i need to close safari, but I dont have it open! I have a MacBook Air

If  you're being prompted to close the browser, but it's not open some other process/app, identifying itself as a browser, is using Flash Player.  A work-around is to restart your system, making sure the 'Reopen windows when logging back in' check-box is NOT selected, and making sure any app that may use Flash Player is NOT running when you try to install Flash Player.