It could be a goo idea to reboot the Ubuntu VPS server once a month.
Start the cron editor with the command
sudo crontab -e
Then add the cron command to restart the server
15 15 15 * * root /sbin/shutdown -r now >/dev/null 2>&1
To verify that this works use this command to get the time the server has been up.
sudo uptime -p
This command will tell you when the server was booted
sudo uptime -s