How and where do I enter my spouse's 2017 AGI (when we filed separately: mfs) to the 2018 return when we are filling together [mfj]

If you're filing jointly this year, select the filing status:

1.  Click My Info on the left-hand side menu.

2.  Scroll down to Marital status.

3.  Click Edit to change from No to Yes under Filing jointly.