The link is on the log in page. You can also find the forms by going to Other Tax Situations>>Other Tax forms>>Amend a return.
You should NOT amend your return until it has been fully processed. It is possible that the IRS will catch your mistake and send you a letter of correction letting you know they fixed it and the actual amount you have to pay in. If they don't catch both omissions, then you will file an amended return.