Upgrading suspension fork
A good place to start is looking up the specifications of your current Suntour fork: http://www.srsuntour-cycling.com/components/fork/XCM-27-5-5617.html

The important points are:


Straight 1 1/8" diameter steerer tube
Post type brake caliper mount for 160mm rotor
9mm quick-release wheel attachment


You will also need the axle to crown length to be the same or at least close. The Suntour page does not give it but you can measure it easily. I'm not particularly knowledgeable about MTBs but I am  guessing that most 120mm travel forks for 27.5" wheels will have about the same length.