Hello, how to upgrade to newest bitcoin qt version? I have to download It and install, and what's next?
Yes, just shut down the old version, backup your wallet (just in case) and install the new version.
Will my bitcoins will be still there on the newer version? I am pretty new to this.
Yes, unless something goes wrong. Thats why you make a backup first. I would not expect something to go wrong, but it does not hurt to be extra carefull.
Also everyone says that you have to know your private key.
The private keys (usually more than 1) are in your wallet.dat, you dont need to memorize them.
As I understand there is no 1 private key that you must know, every address has Its own private key and all of them are stored in wallet.dat file? Am I right?
That is correct. What you need to know is your password that keeps the private keys safe. Sometimes people mix that up.