This is a relatively easy thing to do. It’s all screws, no glue or other messy bits to deal with.
If you are considering a Mac mini purchase, check the replacement RAM costs before you order.
UPDATE: As the linked post says, the images are from the 2014 Mac mini, thought the steps are the same. Thanks to Patrick McCarron, here’s a link to another post that accomplishes the same thing, but on the actual 2018 Mac mini.