I have the old 2011 versions I had purchased from GOG of Myst and Riven and downloaded them today on my windows 10 PC. However, the game graphics on both are so pixelated it is hard to see anything. The originals were not like this at all!!

Do I need to download an updated version? Or where oh where can I just purchase the 25th anniversary edition with all the DVD's etc.

Look at eBay or similar sites - because flip culture is a thing, you'll likely find someone selling a copy at some point for a superbly inflated price. If you didn't back the Kickstarter for the DVD edition, that's your only option to buy them.

It's also not going to make any monumental difference in visual fidelity for the games, but I'm not going to get trapped into that argument in this thread.