Upgrade from IMAC IOS 8 OS Yosemite

What year model is your iMac?

see > Identify your iMac model - Apple Support

What macOS is your iMac currently running?

see > Find out which macOS your Mac is using - Apple Support

To find out which macOS version your iMac can be upgraded to,

have a look at > How to get old versions of macOS - Apple Support

Without knowing which model year iMac you own we cannot answer that question. If you don't know, please click the Apple symbol in the upper left of the display and then select About this Mac and it will tell you what model year you own. However if there are updates for your computer before you do ANYTHING back up the computer if you are not already. This will require an external HD such as a OWC Mercury Elite Pro . You can use Time Machine ( Back up your Mac with Time Machine ) to backup, TM is already installed on the computer.