I have a vanilla Minecraft 1.8 server world but when I update the server to 1.9 it doesn't re-build the world properly and I cannot find the server from the game menu. How do I update it and keep the world we built?
Just copy the
world folder from your server folder to another location (if you named him different: just copy the folder where your world is stored).
Make also a copy of the whitelist, banlist and settings file.
Then just make a new server folder. Download the 1.9 JAR file. Start the server. After it is done installing stuff, stop the server. Dont just close it, but type
stop in the server window.
If the server is off, delete the world folder, the ban/whitelist and the settings file from the new (1.9) server folder.
Last step is just putting the files back you backed up.
Hope the problem is fixed for you. If you have any other questions or need more help, leave a message.