1. if you are signed in to your account and go to your post you should see an edit button at the bottom of the post that will let you edit what you have already written.
2. if your numbers don't match you need to find out why. in other words if you start out with the same numbers, do the same calculations with those numbers, then you should get the same end numbers. if you don't, then you need to compare all of your inputs again and you will likely find that one of them doesn't match. make them match what you do and the end numbers should match as well and that is the option you should go with.