operating 10.10.5 Yosemite on early 2008 iMac and need to upgrade OS.

This computer may upgrade only to El Capitan and no further.

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

And in you case choose El Capitan

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install. Make sure to use Only Safari to commence the download as Others may not work. Safari will open a redirect and open the Apple Store where you can commence the download. If an error message appears, that is probably because one is using another Browser.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install. Make sure to use Only Safari to commence the download as Others may not work. Safari will open a redirect and open the Apple Store where you can commence the download. If an error message appears, that is probably because one is using another Browser.

The links for Yosemite, El Capitan, and Sierra on the page are direct downloads from Apple Support and won't open the App Store. Because of this, they're equally likely to work in any web browser.

The particular problem with newer downloads is that Firefox won't pass the link to the App Store. If someone's using Chrome or iCab, or if they get any error message other than iTunes saying the product isn't available, it's not because of the browser in use.

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