how to make a npc walk with a different animation?

This is the basics:-

        -- load it to the humanoid; get AnimationTrack
        currentAnimTrack = humanoid:LoadAnimation("Animation file here")

        -- play the animation
        currentAnimTrack:Play("transitionTime")

for the full script run the project in solo mode, then find your character. in the character will be an Animate script which all character in roblox use in every game. hope this helps