high sierra says updated in app store but is not

i need to upgrade my OS to the most recent version. i have a 2013 mac pro, so it should not be a problem. the app store is for some reason the only way i can find to allegedly download the update. my about this mac bit says i am running 10.9.5, and the app store says high sierra is already installed, and won't let me actually install it. someone give me an answer please so i don't feel like i wasted 5 grand on this stupid iLife paradigm that i am for real quitting when this machine becomes obsolete...