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First of all, go to sources.list using following command.

```bash
nano /etc/apt/sources.list
```

Then, you will see some link there. You will find following line there.

<span id="source">

```bash
deb http://http.kali.org/kali kali-rolling main non-free contrib
```

</span>

For you it may be `http://kali.cs.nctu.edu.tw/kali` instead of `http://http.kali.org/kali`. You can use whatever you want. I like kali official. Write the following line instead of [above line](#source)

```bash
deb http://http.kali.org/kali kali-rolling main contrib non-free
```

Then try to update your system. Did you get any error this time? Very few people see the error again. If you are still seeing the error than try it.

```bash
wget -q -O - https://archive.kali.org/archive-key.asc  | apt-key add
```

Using the above code you are adding a key. Then try to update your system. 

Relevant problem is available in :

1. [Kali Forum](https://forums.kali.org/showthread.php?33399-Cannot-apt-get-update-Err-1-http-http-kali-org-kali-kali-ro http://http.kali.org/kalilling-InRelease)
2. [Unix SE](https://unix.stackexchange.com/questions/421821/invalid-signature-for-kali-linux-repositories-the-following-signatures-were-i)
3. [Unix SE](https://unix.stackexchange.com/questions/421985/invalid-signature-when-trying-to-apt-get-update-on-kali/422184)

[The answer](https://unix.stackexchange.com/a/422184) solved my problem. And, first method is from Kali Forum.