Increase the RAM

Only certain models of the 21.5 iMac can be upgraded and then only by a Technician (or at least someone that knows what he is doing) it is not a user upgrade and from what I have heard can be very expensive. Here is a link that you can use to see if yours can be upgraded Install memory in an iMac - Apple Support

The biggest question that you can ask is it worth it? Even tho it is only a year old depending on the price and model it might be worth in getting a 27 inch with a basic 8 Gig installed and by Ram from another source and do it yourself.

Welcome!

To add to TJisret's comment about "expensive," I asked a local service shop (US Inland Northwest region) what he charges to do RAM upgrades for a sealed 21.5-inch iMac and he said they have to charge a minimum of two hours labor at US$75/hour to perform the upgrade.That is another $150 on top of the cost of RAM and reseal kit. Plus, if you are doing this because your computer feels "slow," there is a strong chance that would be a waste of money.

If your want 32GB RAM because you computer is:

• • slow to boot, and
  • slow to open apps,

that is likely not a RAM issue but rather the slow mechanical hard drives Apple installed in 21.5-inch Macs from 2012 through 2019. On performance tests they score SLOWER that the mech hard drives Apple installed in entry-level 2011 21.5-iMacs. Even the Fusion drives (hybrid) are not all that fast

So if you can tell us why you would like to increase RAM, this conversation might take a different solution path.

For real speed and flexibility you need a 27-inch iMac so the RAM is user upgradeable—as Roger wisely points out—and the biggest factory solid-state drive you can afford. The later is what make an an iMac speed demon. The 1TB factory SSD in my 2017 iMac 5K posts read/write speeds over 20X faster than our 2011 iMac with a factory mech drive.

choosing the model on crucials site

https://www.crucial.com/compatible-upgrade-for/apple/imac-(retina-4k--21*5-inch--2019)-imac19-2

suggest it's non upgradable memory

Only the 27 inch iMacs have the facility to upgrade the memory.