Log into your modem and the option to change dns should normally be on the first page itself if not find the option where you can set your static ip and dns and on that page leave the ip thing untouched and just click on use dns ( untick the option which says use isp provided dns and click the other one)
Now click on apply and you are done just reboot your modem and you are good to go.
This is not going to increase the speed of your connection but it might give you a bit better browsing experience.
Generally speaking your isp's dns settigs should work the best for you but if for some reason they dont then only resort to open dns
