I want to upgrade my OS from 12.04 to 14.04. I found how to get it. But I've one more question. What would happen if I gave the command to upgrade, it starts but after downloading some or many file my internet connection is lost? An I still be able to use 12.04? And what about downloaded files of 14.04. They would still be on pc or I have to download them from the beginning?
About the downloaded files, they will be saved when the connection lost. You can redo the upgrade and this time it will resume the downloading. You will not have to download whole data again.
From what I know the update process is as folows:
In other words, if your connection is interrupted, nothing will change to your system because the first step is not finished. Step two is executed if and only if step 1 was successful.
This is the upgrade manager you'll see. Note how first the new channels are fetched, then the new pacakges, only then to proceed to install them.
Just before the actual fetching of the packages starts, you will get the following dialogue box which confirms my answer:
Note: You should not be worried so much about your connection, but about making sure your power source stays alive. If you switch off your computer or your power source whilst the update is ongoing, then your system will be messed up.
Also as a general note: always keep two external backups of all your files before you upgrade and do not upgrade a production system without having a large amount of time and knowledge about what to do if something in the upgrade did not go as expected.
For the rest, welcome to 14.04.