This notice comes to you after the IRS has made the changes to your return that you have requested. The notice will explain what changes were made to your return and what you now owe as a result of said changes. Generally, the CP22A deals with changes made by sending in an amended return for that tax year.
If you did not request to have any changes made to your return, you can contact the IRS directly via phone. Make sure to have the letter and your return easily accessible as you will need them to answer any questions. IF you decide to appeal the changes, you have 60 days to do so.
If the letter is correct and you did request those changes, simply adjust your copy of the return accordingly to reflect the correct return the IRS has on file. You will then send any payment due as a result of the requested changes.
If you need to make additional changes that were not covered in the CP22A, you will need to file a 1040X, Amended Individual Income Tax Return.