I used the command nano /etc/dhcp/dhclient.conf. I scrolled down and removed the "#" before the "prepend domain-name-servers" and changed the DNS to two new DNS I wanted to add ( and Did CTRL+O, Enter, and used the command service network-manager restart.
After that, I used the command cat /etc/resolv.conf in order to check the changes and to check if the two DNS were added. Unfortunately, the only thing it appeared in the terminal was:

# Generated by NetworkManager

How can I fix that in order to get the two DNS in the cat /etc/resolv.conf ??