Since inventory flows from one planning period to the next, BOP for the first week is coming from EOP for the past year and you cannot automatically override BOP for week or month 1. If you want to update the BOP values for the first week, please see the steps below:
Go to previous plan year
Unlock previous plan data if needed
Update the EOP value & save
That should update first BOP value for the year
Here is a quick video for how this all works in action.
*Please note the Financial Planner module has since been renamed Merchandise Planning