Where can I manually change the "net employment income" number?

Sorry, I meant to ask about "net self-employment income". Where is that entered and can it be changed? Thanks.

Other than if you have a T2125 which you can edit, there is no way to make a change.

Your net income is a summation of a number of income sources so you will not be able to just change the net income amount. That is, you will need to review/change the other income inputs if your net income amount is to change.