Yes you can upgrade your late 2014 iMac to Big Sur, please read the instructions here,
open your Safari browser and click on the link below
How to upgrade to macOS Big Sur - Apple Support
Although Apple say you can upgrade directly from Yosemite to Big Sur I would advise against doing that.
I would recommend doing an intermediary install of macOS High Sierra.
Part of the installation process for the new macOS's from High Sierra onwards was the conversion of your
HFS+ file system to the new APFS fits system. There have been reports when installing these newer OS's from
older OS's like Yosemite that the installation can hang whilst converting the file system.
Installing macOS High Sierra will provide the necessary firmware to make this conversion smoother.
For macOS High Sierra,
Open Safari and click on this link,
How to get old versions of macOS – Apple Support
Go to the Download OS section and click on macOS High Sierra.
Before upgrading to any OS make a full back up of your Mac.
Check that third party apps you use have updates available for the new OS.
https://roaringapps.com
Check that your printers/ scanners etc have updated drivers and software available for the new OS.
Connect your mac to your router via cable rather than rely on WiFi.
Do not use your mac while it is downloading
and do not use any other devices that share the same internet connection.
If the App Store says ‘Open’ then it is detecting a previous download of the install app
go to the Applications folder locate Install macOS ******.app.
Move it to the Trash, empty the Trash.
Restart, click on the link again, the App Store should say ‘Get' or 'Download'.
If you are considering upgrading to Catalina or Big Sur be warned they do not support 32bit apps.
Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,
some updates may be free, some may be paid. Use this app to discover which apps you have
are 32bit, https://www.stclairsoft.com/Go64/ then contact the developers of the software to see
if they are offering 64bit updates.