hi i need to upgrade the software on my iMac its early 2008

Pre-Installed MacOS:X 10.5.2 (9C2028)Maximum MacOS:X 10.11.x*

Details:*This system can run the last version of OS X 10.8 "Mountain Lion", OS X 10.9 "Mavericks", OS X 10.10 "Yosemite" and OS X 10.11 "El Capitan" -- if upgraded to at least 2 GB of RAM, including booting in 64-bit mode. However, it does not support OpenCL nor does it support AirDrop, AirPlay Mirroring (which also requires a 2nd Gen or later Apple TV), Power Nap, or other advanced features. It is not capable of running macOS Sierra (10.12) or subsequent versions of the macOS at all.

Please note that OS X "Lion" 10.7 and subsequent versions of OS X are not capable of running Mac OS X apps originally written for the PowerPC processor as these operating systems do not support the "Rosetta" environment. To run PowerPC applications on this Mac, it will be necessary to use Mac OS X 10.6 "Snow Leopard" or earlier.

Also see: Which Macs are compatible with OS X El Capitan (10.11)? What are the system requirements? Which Macs support which features?

iMac 20-Inch "Core 2 Duo" 2.4 (Early 2008) Specs (Early 2008, MB323LL/A, iMac8,1, A1224, 2210): EveryMac.com

What version on the Mac OS are you running now?

Your profile states you are on Mac OS X 10.11 El Capitan that is the last OS X your mac can run.

It cannot be upgraded further.