'change' is much more common than 'alter'. 'alter' is a higher register, but more specialized. 'change' is also common as a noun. I need to change to my pants. (take off the pair I'm wearing, and put on another) I need to alter my pants. (take them to a tailor to have their size adjusted) I've changed. (yes) I've altered. (no, alter needs an object) We need to alter the contract before the close of business. (make specific changes of certain points. 'change' would be ok but less precise) The change will be good for her. The alteration will be good for her. (no)

Hello! I agree with what Teacher Mike has said and I would like to add that if you were to speak about a vehicle "altering" it's course or direction it could mean that the direction is slightly different. but If you were to speak about a vehicle "changing" their direction it could be a slightly different direction or it could mean that the vehicle is moving in completely the opposite direction - the same cannot be said for "altering" the direction. I hope that helps!

Change and alter can have exactly the same meaning, but in general, to alter something is to make a small but significant change. Change on the other hand can mean make a small change in something, or a very large change in something, or it can mean replacing one thing entirely with something else. So change covers a larger range of meanings than alter.