How do I upgrade iMac from Yosemite 10.10.5 to a later version of macOS?

Without knowing the model year of the iMac we cannot help you. It is possible your machine has the last version of Mac OS that will run on it however with Yosemite I think you can upgrade those machines to High Sierra 10.13.6. Once you know the model year of the computer (Look in About This Mac under the Apple symbol in the upper left of the screen) then please use:

https://www.macworld.com/article/673697/what-version-of-macos-can-my-mac-run.html

Once you know the latest version of Mac OS your machine will run, back it up using Time Machine before moving forward to safeguard your data!!!!!!!!!!!!! This is a critical step, do not ignore it! Then we can tell you how to upgrade to that version of Mac OS.

NOTE: If the computer has not been turned on for several years it may have a dead VRAM battery, you will know if it does not boot up. If that is the case please take it to an independent Apple Authorized Service Provider to have that replaced, it is not user serviceable.

For a list of iMac models and the Latest compatible OS for each,

see > Identify your iMac model - Apple Support

Yes, more details are important here for a computer model that has been in continuous production since 1998.

However, based on OS hardware secs, we know that any Mac now running OS 10.10 can be upgraded to 10.11 "El Capitan." The iMac must be LATE 2009 or newer to upgrade to OS 10.13 "High Sierra," and 2012 or newer to go higher that 10.13.