How do I change to Google DNS?

[#1]

Do I have to do this in the router or can I do it from the OS?

[#2]

Lets start with what router you are using or what OS are you using?
Edit... If you know what to do you can do It from either the router or your OS.

[#3]

Quote from: reetpetite

Do I have to do this in the router or can I do it from the OS?

Either is possible.

[#4]

The procedure for the TG585v7 and v8 is explained in the Community Support Library article How to change the default DNS Servers in a Thomson Speedtouch Router.