Can i upgrade my imac 2020 27" ram memory

Yes. See: Install memory in an iMac - Apple Support

and

https://eshop.macsales.com/item/OWC/2666DDR4S96S/

and

https://eshop.macsales.com/shop/memory/imac-2019-27-inch

Also note the following from Install memory in an iMac - Apple Support

For optimal memory performance, DIMMs should be the same capacity, speed, and vendor. 

and

Mac (Retina 5K, 27-inch, 2020) DIMMs have a notch on the bottom, slightly left of the middle. If your DIMMs are mixed in capacity, minimize the capacity difference between Channel A (slots 1 and 2) and Channel B (slots 3 and 4) when possible.

You can install as much as 128 GB of RAM (4 x 32 GB) in that iMac.

You may want to check the Memory Pressure graph in Activity Monitor while running a normal workload. On the graph,

• Green indicates that your system has enough RAM, relative to what it is doing
• Yellow indicates that the amount of RAM is marginal
• Red indicates that you don't have enough RAM and that this is likely hurting performance

The way that I read it, your second option above would be preferable -- Channel A (slot1 32Gb and slot2 16Gb ) and Channel B (slot3 32Gb slot4 16Gb )