It depends. What does the letter say? Many times it is a notice of correction and the IRS is making the change, in that case, if you do not agree with the change you need to respond. If you do agree with the change, then you do nothing and they will take care of it. It may result in you having to pay more taxes or sometimes a bigger refund. If you have to pay more, then you will still need to make the additional payment.
If the letter says to amend, then you would need to Amend Your Return.