Is there a way to upgrade OS for 27" iMac (late 2009)

All is not totally lost.

Yes, the user will be putting some money in the to coffers of Apple

But at the same getting a new or newer machine

If the user wishes to explore the Apple Trading In option where the Value of the exiting machine could be used against the cost of a new Apple Computer 

Or Shop Refurbished Apple computers at an Apple Reduced Cost

Have used both previously and have been satisfied each time.

Personal aside on above.

Traded in a Late 2012 iMac for some $$ versus the cost of New Apply Silicon machine

Traded in Intel 2018 MBA versus cost of M1 MBA

Traded in iPhone 12 for iPhone 14

All good here

CherylMcL wrote:

REALLY frustrating! Another place where corporations put profits over people.

I imagine people who bought a computer today that had the same technology, hardware and software as computers from 13 years ago would be even more frustrated.

CherylMcL wrote:

REALLY frustrating! Another place where corporations put profits over people.

Ok.

In 2009 your iMac cost about $1700, reasonably configured. That's about $2.50 a week, even if you were to value it as $0 today. Tell me again how Apple values profit over their target demographic market, who happen to be the wealthiest 2% of people on Earth and think nothing about dropping eight bucks a day on a cup of coffee.

No. sorry. That’s as far as that Mac can go.

Believe us when we say you would not be happy running the latest version of macOS on thirteen year old hardware. Your old iMac would slog along like a Model T Ford and be nearly unusable.

Actually it wouldn't work. For one thing a 2009 Mac does not support Metal graphics.

Since you now know that Mac can't be upgraded past High Sierra, please describe the difficulties you are encountering. There are likely to be some workarounds that can extend its life a little more.