1) Yes, if you have already filed your 2018 Tax Return, you will need to Amend it. You can click here for instructions on Amending your tax return: How to Change a Tax Return You Already Filed.
2) You should report the full amount of your over contribution, $772. Please follow the instructions in this link to report your over contibution of $772 as wages on your 2018 Tax Return: How to report excess 401(k) Contribution
