Put the guitar "on" like you normally would - with a strap on it, put it over your shoulder, like you're going to play it. Then put your right hand on top of the top horn (the "wing" that the strap is attached to) and your left hand around the neck at the nut/headstock. Gently pull up on the headstock while pushing down on the horn. Then set the guitar down and gently tighten the 4 screws holding the neck to the body through the neck plate with a #2 or #3 Phillips head screwdriver (whichever fits better).
The neck is bolt-on, and it's "kicked" a little. You just need to rotate it back. You will need to retune.