I have a 2009 mac desktop umm i was going to download iMovie but it says i need to down load 10.13.6 currently i have 10.11.6 and i go to download sierra i believe but it says in my app store there are no updates available at this time. so What do i do? i currently have os x el captain . i know a little about computers but not enough to know how to fix this and i am not going to pretend how to fix this. please help thanks.