You can update the nameservers for your domain right through your DotBlock client area:

https://billing.dotblock.com/clientarea.php

Once logged into the client area:

Domains > My Domains > Select the drop down next to the domain you wish to update > Manage Nameservers

From this interface, remove the existing nameservers and add the new nameservers.

Once that is complete click "Change Nameservers"