which build should I download after I finished confronting richtor on paradino?
(I don't want to lose any of my save files on the game)
0.21.1 is the most recent public version of the game. If you're playing that, you've already reached the end with Richtor's fight. If you wanna support Spedumon on Patreon, the latest version there is 0.81 (the Patreon version has a different numbering system than the public version, it's not 60 versions behind). Otherwise you gotta wait til November for the next public update.
In order to keep your save files, you must copy the entire "save" folder within your current game's folder (which is inside the "www" folder), and use that to replace the one in the updated version.