Hello, @amirehmarwan

I believe that there are 3 things that you need to do:

1 Use the following command:

sudo hostnamectl set-hostname hostname

2 Update your hostname in your hosts file:

vim /etc/hosts

3 Find the references to the old FQDN and update them to the new one.

Update the cloud.cfg file. Edit the following file:

vim /etc/cloud/cloud.cfg

Find the reference for preserve_hostname and change it from false to true.

That is pretty much it, you can try to reboot your server and then test the change by running only the hostnamectl command.

Hope that this helps!
Regards,
Alex