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root@kamil:~# uname -a
Linux kamil 4.12.0-kali1-amd64 #1 SMP Debian 4.12.6-1kali6 (2017-08-30) x86_64 GNU/Linux 

root@kamil:~# apt-get upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

root@kamil:~# sudo apt-get upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

root@kamil:~# apt update
Ign:1 http://security.kali.org/kali-security kali/updates InRelease
Err:2 http://security.kali.org/kali-security kali/updates Release
 404 Not Found
Ign:3 http://http.kali.org /kali InRelease
Ign:4 http://http.kali.org /wheezy InRelease
Ign:6 http://http.kali.org/kali kali InRelease
Err:7 http://http.kali.org /kali Release
 404 Not Found
Err:8 http://http.kali.org /wheezy Release
 404 Not Found
Err:9 http://http.kali.org/kali kali Release
 404 Not Found
Get:5 http://ftp.yzu.edu.tw/Linux/kali kali-dev InRelease [30.4 kB]
Err:5 http://ftp.yzu.edu.tw/Linux/kali kali-dev InRelease
 The following signatures were invalid: EXPKEYSIG ED444FF07D8D0BF6 Kali Linux Repository <devel@kali.org>
Reading package lists... Done
E: The repository 'http://security.kali.org/kali-security kali/updates Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://http.kali.org /kali Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://http.kali.org /wheezy Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://http.kali.org/kali kali Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://ftp.yzu.edu.tw/Linux/kali kali-dev InRelease: The following signatures were invalid: EXPKEYSIG ED444FF07D8D0BF6 Kali Linux Repository <devel@kali.org>
E: The repository 'http://http.kali.org/kali kali-dev InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

root@kamil:~# apt upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

As you can see, I installed Kali on computer last year. When I ran uname -r, I saw that the version of Kali is old.

What can I do to upgrade?

Before running # apt upgrade, You need to run # apt update in order to check for updates.
In case # apt update will not show any possible updates, please check /etc/apt/source.list (# nano /etc/apt/sources.list) and correct it.