What if you filed your return and then another w2 comes in the mail? If i already submitted my return, how do i go about entering this w2 i wasn't expecting?

If you have e-filed your tax return and need to change the information on the tax return then you will need to wait for the tax return to either be accepted or rejected by the IRS.
If the tax return is rejected then you can make your changes and re-file the tax return.
If the tax return is accepted then you will have to amend the tax return to make your changes. An amended tax, Form 1040X, return can only be printed, signed, dated and mailed by you to the IRS, it cannot be e-filed. The IRS will take up to 4 months to process the amended tax return. Wait until you have received the tax refund or the taxes owed have been paid and processed by the IRS before amending the tax return.

Thank you for your clear answer. I appreciate the time you took to answer my question.