How can I upgrade from my OS X Yosemite?

High Sierra is the highest version supported on that iMac.

A direct upgrade from Yosemite to High Sierra is supported.

Click here for details: How to upgrade to macOS High Sierra - Apple Support

Have a backup prior to the upgrade.

From the menus, 🍎 > About This Mac

Post the Mac model and year.

From Yosemite, you’ll be able to get to at least El Capitan:

How to upgrade to OS X El Capitan - Apple Support

You might be able to upgrade further.

How to upgrade to macOS High Sierra - Apple Support

How to upgrade to macOS Mojave - Apple Support

Maybe as far as Catalina, though that’ll require all your apps to be 64-bit:

How to upgrade to macOS Catalina - Apple Support

Have a complete and current backup before any upgrade.

With the model and year, we can better discuss and tailor your options and choices.

What model year is your iMac (from Apple menu > About This Mac)?