If I remember correctly when you download and Install it there is a point where it asks if you want it to be the 'default' and you just click the button.I keep both on my machine in case something won't open in Foxit but that has not happened yet.
On my laptop Foxit is the only reader I have and it works just fine by itself.
I use it for proofreading the various jobs I work on and everyone that gets my mark-ups says they like it better and eventually they convert to it!
~~~GB~~~