How do I automatically start Ubuntu Update Manager on a particular time of the day and make it download updates ?
Similarly, do I automatically update installed softwares (Ubuntu Software Center) on a particular time of the day and make downloads?
You can either set apt-get to update and upgrade in cron.daily or you could simply use cron-apt. There is also an unattended-upgrades package but I haven't used it.
Unless there is a reason that you only want to work with the Ubuntu Update-Manager/Software-Center, although I can't see why.
It's an issue that has been answered in various locations, so here are some references:
You might want to take a look at unattended upgrades. While that doesn't say anything about when they will take place, another good howto can be found here which works without additional software, and to me looks more like a direct answer to your question. Though you might want to substitute the cron-part from "daily" to something more precise. You might heck this Cron Howto for details on how to handle that.
Install cron-apt:
sudo apt-get install cron-apt
Edit config-file:
sudo gedit /etc/cron.d/cron-apt
comment the line: (put '#' in front of it)
0 0 * * * root test -x /usr/sbin/cron-apt && /usr/sbin/cron-apt
Add these lines to the end of the file:
Every day (midnight)
0 0 * * * root test -x /usr/sbin/cron-apt && /usr/sbin/cron-apt
Save the file.
For more information see: https://help.ubuntu.com/community/CronHowto