Do not attempt to modify your reservation online since the only way to do that is cancel existing reservation and rebook a whole new one and you can end up with no reservation if you try that because your new booking is subject to availability when you try to do it.
To do what you want to do, you have two options. If you need it done quickly, call MS when it is open and have them drop a day and add a day (if it is available) and you do not lose the rest of your reservation during that process. Alternatively, you can go to the website and after you have signed on, click on Help & Contact in the upper right corner, then on the next page click on the envelope that is under "Contact Us." Then send an email to MS making your specific request. Make sure you include your reservation number, the days you have now and be prercise in describing exactly what you want done telling them which night to drop and which to add. Note, when you send the email you will get a form response acknowledging receipt and telling you that they will get back to you within five days. Until recently that was two days and these days it is often six or seven, but they will send you an email telling you that the changes have been made and then you can go online and verify.
