I need to upgrade my OS

See if you can upgrade to 10.11, 10.12, or 10.13, see sys requirements...

https://support.apple.com/en-us/HT206886

https://itunes.apple.com/app/os-x-el-capitan/id1147835434?ls=1&mt=12

http://updates-http.cdn-apple.com/2019/cert/061-41424-20191024-218af9ec-cf50-4516-9011-228c78eda3d2/InstallMacOSX.dmg

https://support.apple.com/kb/sp742?locale=en_US

https://itunes.apple.com/us/app/macos-sierra/id1127487414?ls=1&mt=12

https://support.apple.com/kb/sp765?locale=en_US

https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12

You will get faster and more specific help if you post your iMac's sub-model, form "About this Mac":

Early iMscs have limits on what OS level they can handle. For example, no iMac older than 2012 can install Mojave 10.14 or Catalina 10.15.

Making any headway?