^^^That is called a 3-piece crankset (right crank arm, bottom bracket, left crank arm) with a fixed non-removable chainring that is swaged to the crank arm. If you want to change the chainring size, you will need to buy a crankset that has a chainring that is bolted to the crank arm spider. You will also probably need a different bottom bracket with a spindle that matches the crank arms for a proper chainline.