how do i update from High Sierra to Big Sur

Check the compatibility section of How to upgrade to macOS Big Sur - Apple Support and if compatible, use those instructions.

Uncompliance - uncomprehensive:

a) Mac OS High Sierra cannot be updated to a higher version 14.1... something

b) the excel won't go further and requires an update to this precise higher version of Mac OS

So, the dilemma is, that neither an update of the excel nor of the iMac is attainable.

Might it be that Big Sur would solve the problem? - Perhaps, but it isn't compatible to this 10.13.6.

Please explain what I should do to get these essential updates installed?

Sorry but macOS 10.13.6 is the highest possible upgrade for a late 2009 iMac.

The only option I am aware of for Microsoft Office is to continue to use Office 2016 -- see: https://support.microsoft.com/en-us/office/end-of-support-for-office-2016-for-mac-e944a907-bbc8-4be5-918d-a514068d0056

Thanks,

Well, that's what the ICT companies do - valid harware is no good for new titter-tatter software.

Have ever tried to tell that to car owners? "You just go on the old clumpy paved-wrecked road. This new app is only for those who have more up-to-date cars"...

Anyways, it's nor long ago a valid liquid pump control unit was still running on WinXP.

Have a nice weekend. We just came from a Finnish sauna so I'm not so ****** off.

So, You got Your credits for this. The nut behind the steering wheel didn't.

Tanks and have a good year 2021!

What type of Mac do you have (MacBook Air, MacBook Pro, iMac, etc) and what is the model year. Both of those are in Apple menu > About This Mac.

iMac Late 2009, macOS High Sierra, processor 3,06 GHz Intel Core 2 Duo, 8 Gt 1067 MHz DDR3

Look into the Free LibreOffice, it handles MS Office files like Word & Excel…

https://www.libreoffice.org/