How do update the imac operating system?

Depending on if your Mac hardware could run a later system, there is a

separate link within this site page, along with some other information:

• How to download OS X El Capitan - Apple Support

//support.apple.com/en-us/HT206886

Hopefully after you've determined if the machine itself has sufficient

resources in present hardware configuration to run later OS X, it'll

work out for you. ~ The memory (RAM) may need upgrade to more;

and also the hard disk drive (storage) may need a larger replacement.

And these should be correct specification of quality to suffice.

An example of hardware for upgrade comparison: [ MY UPGRADES ]

Good luck & happy computing! 🙂

You may need to try and see if "Software Update" within your running Snow Leopard 10.6(.8)

system can get one of the few remaining updates. The one(s) missing may be what allows a

transition to get later download upgrades from the Mac App Store or via linked iTunes page.

And see if there's a later iTunes app for your old Snow Leopard 10.6.8 v 1.1 once updated.

{IF you had a previous purchase or major upgrade/download in your App Store listing, that

may be available to use via your AppleID & Purchases, in another supported Mac product.

But only if the intended Mac product was already able to access Mac App Store.}

Looks as though your Snow Leopard may be missing one of the last two/three updates.

Mac App Store Update for OS X Snow Leopard - Apple Support

This update ensures future compatibility of the Mac App Store included with OS X Snow

Leopard, and is recommended for all Snow Leopard users. For more ...

https://support.apple.com/kb/DL1860

• Mac OS X 10.6.8 Supplemental Update

//support.apple.com/kb/dl1429?locale=en_US

• Download Mac OS X 10.6.8 Update v.1.1(2)

//support.apple.com/kb/DL1400?locale=en_US

• Mac OS X 10.6.8 Update Combo v1.1 early version?

//support.apple.com/kb/DL1399?locale=en_US

You've not found a link for Yosemite's full installer.

The update to the fully installed base 10.10.x could only be used in that main OS X.

{That's not been in App store in a few years. Except via a 'previous purchases'

in ones App store, after gaining access via 10.6.8+ only if you got/bought it.}

So if the Snow Leopard 10.6.8 had not been updated prior to attempting to gain access to

the Mac App store, it may not have fully been able to interact with the content pages there.

In any event...

Good luck & happy computing! 🙂