Yes, you can manually edit both informations.
To modify only your DNS, you should edit the file
/etc/resolv.conf and modify the lines which begin with
nameserver. Like this (here, it's with google's DNS) :
What do you mean by modifying your DHCP address ? You mean the IP address the DHCP which has been given to you ?
If so, there are two steps :
- Disable the DHCP to avoid overriding of your parametters
- Modify the address itself
These depends on your system settings (do you use NetworkManager, on which distribution are you, ...).
EDIT : You cannot set the DHCP server IP (or choose which DHCP server you want), because when you connect to a network and want an address via DHCP, you send a broadcast query.There should be at most one DHCP server in a subnetwork (I don't know what happens if there are more than one and there is a conflict).