See this answer from the helpful user Lentilbean, on the first page of Google when directly querying your title:
This is decided by the routing table on your computer. Open a command prompt and run "route print" to see what it looks like. The interface that has 0.0.0.0 assigned to it will be the one used for internet access. If there are more than one with 0.0.0.0 then the one with the lowest metric will be used. One way to force internet traffic to go through a certain interface is to add a route with a low metric for 0.0.0.0 to this interface.
Run "route print". Look at the Interface List and remember the number of the interface you want to create a route for.
Run "route delete 0.0.0.0 if ". This is because you cannot have two entries for the same route on the same interface.
Run "route -p add 0.0.0.0 mask 0.0.0.0 metric 1 if ". The -p switch makes it permanent. Metric 1 will give it the lowes cost, hence it will be the preferred entry to use for requests to addresses not specified in the routing table.
Btw, having default gateways on more than one interface is generally not a good idea.
See the full thread here: https://social.technet.microsoft.com/Forums/windows/en-US/64451b61-b8d6-4e16-a396-ea418753f1c6/force-internet-connection-to-desired-network-adapter
I suggest researching questions such as this prior to asking.