Minecraft – How to update the smp server to a snapshot

The Minecraft server simply uses the current working directory to store its files. Moving the data is as easy as actually moving it and starting the server with a different working directory.

How you do that is mostly up to you. Windows shortcuts allow you to set the working directory, as do batch files or Linux shell scripts. Since you mentioned you're using Ubuntu, this simple shell script should do the trick:

#!/bin/bash
cd /home/user/Dropbox/minecraft
java -Xmx1024M -Xms1024M -jar /home/user/minecraft_server.jar &

However, as BlaXpirit pointed out, Minecraft's server files are frequently written, and Dropbox does store all versions of synced files. Depending on the popularity of your server, this might put unnecessary strain on your network and Dropbox's service. If you just want a backup, scheduling a job to periodically move the server files to your Dropbox folder might be a better idea.

Bukkit does not update for snapshots. You cannot run a Bukkit server on anything except a primary release. Unfortunately, you are out of luck. You'll have to run a vanilla server or just wait for the next full version to be released.