How to slow down an animation

How do I get An animation to slow down, it’s playing to fast. Do I need a script or something?

With animation.speed.
From the Script Reference:

// Make all animations in this character play at half speed
for (var state : AnimationState in animation) {
    state.speed = 0.5;
} 

Animation anim;
private float animSpeed = 0.6f;

anim = GetComponent<Animation>();
anim["name of animation"].speed = animSpeed;

You can decrease the samples in the animation editor.
Generic animations are 60 samples per second. If you decrease the sample rate the time between frames will be longer and also the animation will be slower.

In the animation editor, you can hand-adjust all the keyframes. I don’t know a way to simply lengthen the animation in the animation editor, though. Anyone?

In Animator window, select the state you want to slowdown its motion and decrease the value of Speed attribute in Inspector.