It is simplest to manipulate the routes with the ROUTE command. You can only have 1 default route so you would just put in ROUTE DELETE and ROUTE ADD commands to choose.
Now if only one of these networks has internet you can actually have both active at the same time. You would just put the default route on one and then put in the routes to the other network with route add commands. You can actually use 2 internet connection if you are work hard enough using this method also.