Coming from Amsterdam and going to Le Harve. It appears I need to change trains at Gare du Nord. Do I walk, take metro or taxi?
I'm assuming you're taking a train from Gare Saint Lazare to Le Havre (about 2h10m train journey) but you're arriving at Gare du Nord from Amsterdam aboard Thalys.
After arriving in Paris you'll need to transfer between Gare du Nord and Gare Saint Lazare using the RER E train line (these two major train stations that are only one RER E train station apart from each other).
The RER E train is connected to Gare du Nord (via indoor tunnels or on the street, your choice) to Magenta station (on the RER E line). For the indoor option, at Gare du Nord follow signs for RER E and that will get you to Magenta station. For the street option, Google maps works fine.
Once you're at Gare Saint Lazare you'll look for signs for "Grandes Lignes" which are all the surface trains to other cities, such as Le Havre.