There is no way to "ugrade" a 2011 Mini to a 2020, you would have to buy a new computer. If you buy new direct from Apple, the cheapest 2020 Mini is $799. Apple has a trade-in program that you can check, but you will get virtually nothing for a 2011 Mini.
Get a new Mac mini with M2 chip from only $49.91 per month. Select a model or customise your own. Buy now at apple.com.
www.apple.com
As usual, you need to shop around. Big retailers often have discounts on the Mini. Here in the US, B&H Photo currently has this same model for $759, for example.
Apple also sells refurbished Mini's that have the same warranty as new. I have seen these as low as $600, but they have the smaller 128gb internal SSD that has been discontinued. They sell very quickly, so you need to constantly check Apple's site and be prepared to buy immediately when you find one. They have a lot of different configurations, and it might take a month to find the refurb you want. Most of the time, there are none listed at all.
Save up to 15% on a refurbished Mac. Tested and certified by Apple including a 1-year warranty. Free delivery and returns.
www.apple.com