If the report has already been submitted, you can't.
After a walkthrough report is submitted, it cannot be modified. This makes fair to both landlords and tenants; neither party can modify the report and both may access it.
If you want to add an item to a report that has already been submitted, simply create a new walkthrough report.
If the report has not been submitted, you can.
If you are the one who started the report (i.e. the "inspector") then simply go to the report and continue where you left off. You can access and modify the unsubmitted report from your cellphone, tablet, or computer.