i would like to update my mac

iMacs from 2008 to 2010 are very very limited by todays standards.. If you want to stay up-to-date you also need to keep your equipment up-to-date too. Personally I buy a new Mac every 7-10 years. You are LONG overdue to buy a newer Mac. If you are concerneed about your old equipment Apple will respnosibly recycles yours at no cost to you.

If you do not have the finances to buy a new iMac there are many other alternative to you including:

• Don't buy an Imac, consider a new M1 Mac Mini, for less than $1000 you can have a new Mac that will far out perform your curent computer(s). These only need an external display (less than $100) and you can use the old Mac's keyboard and mouse.
• Consider an Apple Refurbished Mac, you can get these directly from Apple. Check Apple's home page in your country and look at the bottom of the page to see if they ofer the Refurbished program where you live. If they do, you will find the Mac section. The advantage to these computers ae they identical to a brand new Mac, have the same 1 year warranty as a new Mac, are elibible for AppleCare and can save you hundreds of $$. If decide to go this route, ONLY buy one that has at least 16GB of RAM and a SSD for storage.
• Finally, you can buy used, refurbished Macs but ONLY buy from reputable sources such as www.macsales.com. NEVER buy from E-Bay, an Amazon reseller or other similar site.

Depending on the exact model of your iMac, it could be upgraded as far as:

• Early 2008, Early 2009, Mid 2009 models: OS X El Capitan 10.11.6
• Late 2009, Mid 2010 Model: macOS High Sierra 10.13.6

if you mean upgrading to Big Sur check this list to see if it can.

https://support.apple.com/en-gb/HT211238

Not possible, unfortunately.