Finish the hanging doors if the surface is unfinished wood. You can either stain or prime and paint them.
Open the bi-fold doors to fold them.
Remove the doors by lifting them up to remove the pin at the bottom of the doors from the bracket and pulling the bottom of the door out away from the closet. Slide the doors out of the track on the header. The header is the top of the door frame.
Unscrew and remove the track and the brackets on the floor.
Measure the length of the door frame, and cut the track for the hanging doors to fit with a hacksaw.
Place the new track against the bottom edge of the header, and screw it to the header. When driving in the screws, drive them deep enough that they don't extend past the surface of the track, where they can impede the doors from sliding. You may want to have an assistant help you with this step.
Hang a plumb bob from the center of the track down to the floor. Mark the location of the plumb bob onto the floor, then remove it.
Screw the bottom bracket onto the floor. This acts as a guide that prevents the doors from swinging back and forth.
Install the wheels onto the doors if they're not pre-installed. Follow the manufacturer's instructions for the doors you purchased.
Hang the doors by lifting the wheels on top into the track, and lowering the bottom edge into the floor brackets. Slide the doors open and closed to test them.