Need help getting From OS HIGH SERRIA 2 OS MOJAVE PLEASE HELP

SO Im trying to install a newer OS on my 27in iMac all in 1 it currently is using OS high Serbia and with that OS I am unable to download a bunch of different things that I would like to. Apps that I actually need for work. My iMac is a mid 2010 and it is in good shape but the things that I can't download are causing me problems that I am trying to do in school. Please help or give me instructions on what to do.